That's surprising EDO10. I played it twice on XP with all settings on HIGH and at 1600x1200 but no Anti Aliasing or Anistropic Filtering and at times it really bogged down and I had to revert to 1280 x 1024. On Vista with all settings on VERY HIGH at 1600 x 1200 I had no problems at all. I read that DX10 can use more of the CPU as well as more efficient use of the GPU so maybe with my 8 cores I saw a real difference.
As more and more games come out using DX10 I think Vista is the way to go - but that's just my opinion in response to the OP request.