Well, despite the x1950 PRO being a faster card than any of the three, and being in that price value...I'm gonna say the 7900 GS.
Here's my reasoning...the 8600 GT doesn't have enough unified shaders to compensate for the dedicated-type shaders of the earlier cards...right now, unified shaders are less efficient than their specialized brethren.
Also, the 8600 isn't really going to be your card of choice for DX10 games, because...well...it's not that fast.
The 7900 whomps the 7600, clearly, and thus, should be your upgrade.
well see i have a sli motherboard which is why i opted for a nvidia GPU. are there any new cards coming out christmas time? a new line up of cards or refreshes? i was thinking of waiting for the new AMD chips to come out and maybe switch over to ATI or Build a new Intel Machine.