You may want to try running Doom 3, if you haven't already. I was running my 9800 Pro at 425 for awhile, but in Doom 3 I had to drop it down to 412 to get rid of the white dots. If you can run 432 in Doom 3 with no dots, than that is a way good core OC!
For some reason 9800 Pros Artifact badly in D3. Even at stock speeds and with a chilled water Maze-4, my old 9800 Pro had some slight artifacting in D3. All other games were artifact free up to 493/378 mhz. I now run a X800 Pro flashed to XT PE at XT clock speeds with no hint of artifacts in D3.
if its the good samsung ram you will be able to hit 390 (ddr 780) without much problem. on a chilly night i can hit >400, but i would never run my card with my ram at that speed for more than a few mintues.