you can choose the DX level by adding -dxlevel "80" in the console or in the advanced start up options for dx level 8.0 or "81" for 8.1 and" 70" for dx 7.0 and so on.
You should be able to play just fine. I have seen it go just fine on my old 1.4 Tbird system with a Radeon 9200 as well as my laptops 64MB Radeon 9000.