I would take the GF7900 in your case, even if it's not a dx10 card,
it has a much better bandwith and technical capabilities:

http://www.nvidia.de/page/geforce_7900.html

compared to

http://www.nvidia.com/page/geforce8.html

If you go for DX10, dont go for something below a GeForce 8600 GTS.

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