Under a grand huh? Ok I'm going to operate on the asumption that you will be re-using your existing CD / DVD drives for now then. I'm also going to focus on motherboard, processor and video card, and reccomend a minimum of RAM since you can easily upgrade that later on.
Motherboard: Asus P5n32-E SLI $299.00
This motherboard is the current top of the line for Asus in my oppinion on the Intel side. It is SLI capable, but you do not have to run it in SLI mode initially. Adding a second video card later on is not a problem (no windows reinstalls or anyhting stupid along those lines) Regardless of if you end up going SLI or not, I would get this board. Keep in mind the motherboard is the most pain in the *** component to replace, so i would advise going with the best board you can afford.
http://www.asus.com/products4.aspx?l1=3&l2=11&l3=397&model=1459&modelmenu=1
http://www.newegg.com/Product/Product.asp?Item=N82E16813131073
There is also a super high end variant of this board called the striker extreme. it retails for $399 though, so thats a bit out of the budget.
Ram: $99.00
I usually reccomend Corsair ram. But they are expensive and this is a budget build. So, generic ram, Two 512MB DDR2 800 sticks, for a total of 1GB. This will leave you two empty ram slots. You want to upgrade in pairs and keep the Ram type pretty much the same. Nothing wrong with this ram, it's just not a name brand
http://www.newegg.com/Product/Product.asp?Item=N82E16820161067
Processor: $308.00
On your budget, about the best processor available is going to be the Intel Core 2 Duo E6600 Conroe 2.4GHz. This is a true 2nd Generation dual core processor, and performce on par with the older Single core 3.4 Ghz processors in most situations, including gaming. If you find any other budget money available, You may want to go with a better processor
http://www.newegg.com/Product/Product.asp?Item=N82E16819115003
Hard drive: $150
For a gaming PC I like to reccomend a dual Hard drive Raid 0 array, but I don't think we can squeeze one in on this budget. So instead i'm going to reccomend a single 74gb 10,000 RPM Western Digital Raptor drive. If that ends up being not enough space you can also add a secondary 300-400 gb drive later on.
http://www.newegg.com/Product/Product.asp?Item=N82E16822136033
You could also skip the 10,000 RPM drive and go with a 7200 RPM drive for around $75.00
http://www.newegg.com/Product/Product.asp?Item=N82E16822144701
If you are thinking about going SLI or getting a high end video card, you'll want to reserve $100 for a 500-600 watt power supply.
Video card:
As far as graphics cards go, the current top of the line is the EVGA Geforce FX 8800GTX. Two of those in SLI mode and you would have the most insane gaming PC on the surface of the planet. At $650 per card though, they are not at all cheap.
http://www.newegg.com/Product/Product.asp?Item=N82E16814130072
Next step down from there would be 7950 gx2 $450
This is what I currently run. The cards are actualy 2 cards in onw. SO you can run quad SLI if you have two of them. Two 8800's are going to be faster than 2 7950's, even with quad SLI.
http://www.newegg.com/Product/Product.asp?Item=N82E16814150160
After that, there is the 7900gx. The individual card core is faster than the 7950, but the 7950 has 2 of them, so it is overall faster. This is still a very good card, and this particular one is only $199 after rebate.
http://www.newegg.com/Product/Product.asp?Item=N82E16814150195
SLI is cool, but i'd go single for now and upgrade later on this budget.
So if you go with the 7900gx card and the slower hard drive, you could put that together for around $908. You would also need a case and power supply if you are not going to reuse your existing ones. I can reccomend a good case if you like. The Raid 0 array really does help for gaming, specifically for Zone load times, but it is expensive to get into and you would need two of the 74gb 10,000 drives (or 2 of the even spiffier but twice as costly 150gb 10,000 drives). It's one of those decisions that whatever way you decide, changing your mind later on will require a full windows reinstall.
You might also consider building in stages. Get the higher end components and processor, but get the cheapest pci express video card you can find. Live with it for a few months and then buy a high end card? I've done that in the past and it works well in the end.
The new quad core intel processors are coming out very soon as well, so there may be a price drop on the core 2 duo processors. that motherboard will suppors either type.
Hope that helps!
Edited, Nov 24th 2006 5:10pm by Kaolian