Mine. I'll sell it to you for $1200 and I'll use the money to build something more inferior, because mine is simply the best.
You are probably looking at an i5 machine if you can't recycle anything (i.e. case, power supply, etc...). If you want a single card gfx card you probably are looking at a ATI 5850 for about $300. If you want to set up crossfire you can do well with two 4870's or two 5770's (4870's for a little more power, 5770's for DX11).
I wouldn't waste my time on duel video cards, if you were thinking that route, I would pay extra for a higher end single card... you end up with a card that has a longer life that way instead of two lesser cards. Power supply... do NOT skimp here... make sure to get one that will cover your power needs for your video card(s) as they have very specific needs these days. Also take into account anything else your running... I tend to go with 500 watt and higher as a minimum, but brand is important too. Next is your video card, pick the one you want. I tend to try to balance cost vs performance, so I dont' spend $800 for a new top of the line one, I instead pay 200-400 for a step or two down from those. Nvidia GTX 295 I think is their high end? I wouldn't go lower then the GTX260 (250 is not much better then an 8800 from what I hear... which is what I run but is a couple years old) ATI I am not sure what their high end card is, as I hate their drivers... My brother got a 4850 I think which was about on par with the GTX260? Next processor and motherboard. Motherboard is based on processor type and options really... you can spend a ton of time researching them, but honestly something in the $100-200 range is going to be fine. Again with the processor I tend to find the best bang for the buck, and for me that was still sticking with duel or quad core. My brother just built a system a couple months ago and went with the duel core E8400 as it was the best bang for the buck still. (sitting around $200 cdn, vs $257 for the I5 which was closer to 300 when he built his) With the I5 being only $57 more I would go for that at this point I think. Next find the RAM that fits your motherboard and decide how much you want. I tend to suggest 4GB no matter what... and if your running vista or windows 7, you can go higher then that if you want.
Starting from scratch, you could put together a good i5 rig for that $1200 you're looking to spend. Here's just a really quick go at it with a little research: Case: Antec Nine Hundred Two CPU: Intel i5 750 MOBO: ASUS P755D Memory: 4GB OCZ Platinum DDR3 1600 Graphics Card: Sapphire Radeon 5850 Power Supply: Seasonic S12D 750W HDD: Western Digital Caviar Black 750GB CD/DVD Drive: Lite-On 24x DVD Writer I dunno if you need a copy of Windows 7 or not. If you do, you can get an OEM copy of Win 7 Home Premium 64bit for $107 at newegg.
The i5 is newer than the i7, but the i7 is the biggest performer. For 1200 bucks you can definitely go i7. Caviar black is a great hard drive. You might consider waiting just a teeny bit of time for the 5890 cards to come out because the prices on the 5870 will drop a little bit once those are on the market and the solve some of their supply problems. I don't recommend the 5850. Also the reason not to get two video cards is simple, a mobo that supports two full PCI-E 2.0X16 slots is twice as expensive as one that supports one, with limited return. I got lucky and got one and it's great but I got it half price. I would have bought the 1 PCI-E 2.0X16 model if I hadn't. Bookmark this article for current comparisons of video cards. http://www.tomshardware.com/reviews/best-graphics-card,2464.html For a DVD drive, I just bought this Sony Optiarc. http://www.newegg.com/Product/Product.aspx?Item=N82E16827118030 I did a bunch of research on MYCE (nee CDFREAKS, remember them?) and it's the shit. I just installed it and it's great. Consider this case, the Cooler Master RC-690-KKN1-GP. I've owned two of them and they're inexpensive, attractive, easy to use and roomy as hell. http://www.newegg.com/Product/Product.aspx?Item=N82E16811119137 For all your questions and nerdy advice regarding computer components, go here and ask the guys at [H]ardforums. They're the best, and the buy/sell is the best place on the internet to buy computer stuff. http://Hardforum.com/ And don't forget to budget 40-60 bucks for a CPU cooler.