I wish the 8600gt was never released, so ppl wouldn't consider buying it. Also, when it comes to pc parts, especially VGA, I don't see how anything could be overkill for too long. My guess is you'll be wanting that 8800 back in 3 months.
as people have already said, the 8600GT's name doesn't do it justice. Compared to other x600 cards, it's significantly slower than the x800 cards in the same series. In fact, the 8600GT is roughly similar in performance to the 7600GT. The card is by no means a midrange card. so in short, just get the 9600GT, or if you wanna go ATI, look into the HD3650.
The 9600 is way better then the 8600. But lets clearify something. The 8600 is way better then the 7600. I have an 8600 gt with 512mb ram in one system. I have windows vista 64bit and 4gb patriot ram with a triple core phenom. I can play Stalker, Crysis, Dead Space and Fallout 3 with 1400x900 and most settings set to high with smooth framerate. Recently I heard a lot of people saying the 7600 wasn't much different. So I put a 7600 (same brand pny with 512mb ram) in the same machine (the 8600 is worth more so I thought sell it and use my old 7600). I had to turn settings down to medium / low and 1024 x 768 resolution and the game still lagged. The 8600 may not be the best card but it blows away the 7600, and it is a mid range card, my games still look better then xbox360 and ps3 with an 8600.