Hello,
Some comments:
1) AoC definitly likes 512MB of texture memory, so dont go for anything below that. I say that because someone mentioned a 128MB card! Stay away from that. I am using a 8800GT (512) and the game runs as smooth as silk on 1920x1200 with almost maxed detail settings and 4x AA. There are many changes implemented lately that allow you to play decently on lower-end cards, but I hate having to turn down those sliders!!!
2) There is currently no SLI support due to the way the rendering engine works. Spend the extra money on a better card.
3) Don't go crazy with DX10. The game looks equally amazing in DX9. I will stay with WinXP for a long time. But anyway every card that is out on sale now supports it, so no worries!
4) 2GB of DDR2 should be OK. More memory always helps the overall system performance, but you will not need it for AoC alone.
5) A fast hard disk (like the Raptor) is going to help alot. Those beautiful visuals are BIG.
See you in Hyboria