Problem is if you're still on an AGP board like me. Ubi went to SM3 on a few of it's games and I don't see them going back anytime soon. Expect them all to be that way. SC was too.
You can buy GeForce 7600 cards that are AGP 8X right now. These support SM3 quite well. Even the GeForce 6X00 cards support SM3. (I played through SCCT using SM3 on a GF 6600).
The GeForce 6600 will run R6V but just barely (frame rates are quite low even at the lowest settings).
We're all going to have to move to the PCI-Express systems eventually if we want to keep playing new PC games. I think the GeForce 8800 cards are PCI-Express only. So the 7X00 line is the last of the nVidia AGP cards, I think.
I have heard of GeForce 7900 AGP cards. But I haven't been able to find any for sale. Anyway the price of the ToL cards (AGP or PCI-E) is still pretty ridiculous (i.e. $300+).
I played all the way through GRAW on an AGP based system. (GeForce 7600GS 512MB). So there is still some room for new games. But the next generation (within the year) will probably not run at all on AGP based systems.
I don't know what DX10 is going to do to us. I already can't go back and recompile some of my code that was written using DirectX 5.