im running on DX10, but not on max settings, im at Very High settings, Ultra High is the highest. I think my graphics card is holding me back. Im going to try to SLI my rig during December because my mom is going to China, and she said she would try to get one for me.
I have my 8800GT OCed, and i get about 25-60 FPS, mostly around 40FPS (average) but thats because i play at 1680x1050 resolution.
I dont think the 1800GTO is DX10 though, and dont think you can run it on very high either, maybe on medium. It depends on the rest of your hardware, and what res you play and if you turn on AA or not.
I dont think i heard of the 1800GTO though, so i wouldnt know the performance output of it.
according to the recommended specs, you need a 512 MB video card with Shader model 3 (NVIDIA GeForce 8600 GTS + / ATI Radeon X1900 +)
Dont let the graphics worry you, just enjoy the gameplay