milnko - That sound card sucks - Believe me, I had one. Get a SB Live! Value, and see things improve. From what I understand, it offloads voices to the CPU to process, and I had all sorts of problems with it. You should be able to get a SB Live! Value for about $30. Also, get the Detonator driver from nVidia instead of using that Asus driver.
I have a PIII 800, 32MB GF2 MX, 256MB, SB Live, and I get 50fps at either 1600x1200 16 bit, or 1280x1024 32bit.