Take that Ram, PSU, and Hard drive out of that POS set up.

Celeron's are the worse, and that's not a good Biostar mobo either.

That GPU is way out of date.

So lets get started, shall we?

If you get a AGP card from a normal retailer you're looking at 280 dollars for the top AGP card, which you can get an 8800 GTS 320 (2-3x faster and direct x 10 compatible) for same price on PCI express.

You're really beating a dead horse by trying to breath life into this system.

I'd start over with a new mobo with some upgradability to it. I'd say go with AMD. You can get a dual core AMD for around 60 dollars which will run anything now days with a decent graphics card (and will be way faster than your current get up) or you could spend a little more and get a 5000+ dual core AMD and be screaming.

Why go AMD? If you're tight on budget, AMD boards are cheaper and typically come with higher quality chipsets then the intel equivalent price ranges.

Find yourself a cheap 1950 Pro PCI express card (which now that you're pci express that 280 price drops to 120-140.)

Make sure your PSU has the pci express connectors required for a pci express card, if it doesn't spend a little more and get a PSU that has two pci express connectors since most future cards will require it.

The rams honestly fine as long as it's same brand so it can run in dual channel mode. Won't need anymore than two gigs for a cheap gaming rig. If you want more, buy another gig (just make sure it's two 512's).

Hard drives... honestly all hard drives really affect is load times so your old hard drive will be fine. I'd just back up everything before you change out mobo's since you'll have to wipe the hard drive.

If you've got the money, buy a cheap enclosure kit for your HD on newegg and use it for data back up and pick yourself up a new hard drive. You can get a decent SATA hard drive for 50 bucks now days.

As far as pentium D goes - don't do it, they run real hot and aren't any quicker than the AMD counterparts. The extremes are still over priced, and the P4's are slower than the single core AMD 64's.

Plus you're going to have to ugprade that mobo because I doubt it's an LGA 775 socket. You're going to want to match up the socket with the CPU.

ATI HD2600XT>8600GT

ATI's cards also use more energy, which means larger energy bill every month. Of course, it's negligable (a few dollars). ATI's cards typically require 2 PCI EXPRESS connectors as well. Honestly I'd stay clear of ATI unless you want a really fast directx 9 card. If you're going to go with the 8600, you might as well go with the radeon x1950 pro since it's a really fast card (faster actually) and it'll still be another 2-3 years before developers even start thinking of just directx 10 only games. Since we really don't know how well the 8600 performs at processing direct x 10 we really can't say it's a viable choice for dx10.

Especially when VGA charts show that the PRO can outperform the 8600 GT by nearly 2x FPS on majority of games out there. http://www23.tomshardware.com/ graphics_2007.html?modelx=33&model1=725&model2=854&chart=279

That dual core is OK. It's a better value to spend a little more and go with a 6320 (double the cache size which is very important) or a 6420. I hear even the 6600's are cheaper now, but spending a little extra won't hurt on your processor.
http://www.newegg.com/Product/ Product.aspx?Item=N82E16819115015

Definately worth it. I'd go with the 6320, great price.

As far as the motherboard goes that is a solid board but you're paying extra for 2 PCI EXPRESS slots that, since you're building a budget system, you probably won't be able to afford soon enough to take advantage of. By the time you would buy two cards, it'll most likely be more viable to upgrade to a new card all together. I can't see too many programs in the future really taking advantage of SLI boards. From what I hear SLI can be a nightmare with some of the current games that support it.

Although it's never a bad idea to spend extra for a good board - which that board you have a less chance of DOA and hick-ups (since reviews from a quick google showed that people didn't seem to have too many problems with it).

The only time your CPU should ever overheat and burn is if you're a complete idiot. Most when spending 100-200 dollars on a processor typically will spend a little extra and get some decent thermal paste and a heat sink, but even using stock fan it'll almost never do that unless it came DOA. Processors DOA is rare.

It's worth mentioning that I only have $400 to spend on this.

The reason I picked the 8600GT is due to it's double lifetime warranty really (and I hear this particular version doesn't make too much noise nor drain much energy at stock settings).

I might get the X1950 Pro if I could find a good one for about the same price.

That might not be such a good idea either. IIRC, all nVidia cards, 7900 and down, do not support HDR and AA simultaneously.

Now I know that he's going with a SLI mobo, and if he gets two 7900s, he'll be able to do both HDR and AA simultaneously... But by themself, the 7900 will only be able to do one or the other.

A lot of people are picky about graphical quality in their games, so I figured I would let you guys know.

Now, about developers and Dx10... I think that they're going to make a full switch sometime late 2008. Not a big issue if you have the cash to buy another graphics card by that time.

And now about the Geforce 8600 and dx10 performance. If you're looking for even mediocre Directx10 performance in games, you're going to want at least $300USD. Benchmarks are out, and the results are not good for ALL Directx10 cards. Maxxed out settings for most games, and the Geforce8800GTX barely puts out 30FPS. If you want Directx10 badly, wait. Wait until the problems are fixed (mostly drivers problems).

[EDIT]: Now, excuse me if I'm wrong, but I thought the ATi X1650XT performed better than the nVidia 7600GT?
http://www23.tomshardware.com/ graphics_2007.html?modelx=33&model1=725&model2=854&chart=279
There are X1650XTs on Newegg that cost less than $100, so it looks like the 7600GT isn't the king of bang for the buck anymore.

I was wrong - I did some more research and found out that with the recent nvidia drivers the 8600 GT is the price per preformance king atm for the mainstream market.

Although I also looked around at Geforce's future and found out that the 9000 series is suppose to launch this November, which if that is true means the 8000 series is gonna have a nice price drop. Especially since the 9200 is suppose to be twice as fast as the ultra. Might be worth waiting a few months.