It gets pretty damn expensive. After struggling to get X working on Moya for a couple of hours last night, I finally broke down and bought a new video card for her this morning. If I remember correctly the card I currently have is probably about 3 years old and it really doesn't like being forced to run the newer versions of X.
So I just bought a card that still shows up in Tom's Hardware guide as a fairly good card... even compared to the newest and meanest (and most obscenely expensive) cards. It benchmarks at about 85% of what the absolute top of the line cards do while being about 33% of the price. Even more promising, the maker just released updated linux drivers for XFree 4.3 today (so I'm pretty damn psyched).
Now I just have to wait and see how long it takes to get here...