Don’t buy the top of the line video card.
Do buy two older GPUs and SLI them. If you’ve got an ATI chipset you’ll need to shop for CrossFire compatible video cards.
Let me crunch some numbers for you:
You can buy a GeForce 8800GTX with 575MHz core clock and 1800MHz memory speed for around $750.
You can buy 2 GeForce 8600GT with 620MHz core and 1600MHz memory clock for a total of around $300. Yes they have less memory but turns out on-video memory isn’t as much a performance gain as processing speed and pipelines. The video cards can only process so much data at one time so having a huge memory buffer waiting to be processed is just a waste of space.
The memory interface of the 8800GTX is wider than the 8600GT but you pay a premium for the latest technology whether there is a clear advantage or not.