Don't worry about DirectX 10.1. None of the game developers will be, because they will aim for systems that are going to make them lots of cash i.e. DX 9.0C.
If they did go DX 10.1 (or DX10) then they would have wasted time,thousands of pounds and resources just for a small market share (though DX10 is becoming viable as more people buy PC's with Vista and DX10 capable cards) and there wont be many people rushing out to buy a DirectX 10.1 Graphics card (that don't exist yet) just because M$ say that "OH LOOK YOUR STATE OF THE ART £460 EVGA Nvidia 8800 Ultra 768MB WONT WORK WITH WITH OUR LATEST GAME API SO YOU WILL HAVE TO BUY ANOTHER"
So I would say go Nvidia 8600 GTS 512MB but before you do make sure that the card
1... is compatible with your mother board
2... your power supply unit has enough jucie for it
3... that it will fit in the case and wont clash with any connectors as some can be quite big (the 8800 Ultra has this problem as I found out the hard way. Its just that I can't fit another one in there to SLI them without ditching a hard drive)
If they did go DX 10.1 (or DX10) then they would have wasted time,thousands of pounds and resources just for a small market share (though DX10 is becoming viable as more people buy PC's with Vista and DX10 capable cards) and there wont be many people rushing out to buy a DirectX 10.1 Graphics card (that don't exist yet) just because M$ say that "OH LOOK YOUR STATE OF THE ART £460 EVGA Nvidia 8800 Ultra 768MB WONT WORK WITH WITH OUR LATEST GAME API SO YOU WILL HAVE TO BUY ANOTHER"
So I would say go Nvidia 8600 GTS 512MB but before you do make sure that the card
1... is compatible with your mother board
2... your power supply unit has enough jucie for it
3... that it will fit in the case and wont clash with any connectors as some can be quite big (the 8800 Ultra has this problem as I found out the hard way. Its just that I can't fit another one in there to SLI them without ditching a hard drive)