It doesn't matter much that your mainboard doesn't have an ATA/66 controller on it. The real spec is in the higher RPM's of the 7200's. ATA/66 versus ATA/33 is not that much of an issue, and the performance diff is not significant at this point in time. I use a Quantum LM 15GB on a udma/33 board, and noticed a big jump in performance over 5400 RPM.
Anyway, if your applying a lot of directx, real-time effects, you may be seeing the limitations of your CPU horsepower as well. But, with your system spec's I would think you should be able to get a stable 8 tracks with few fx on each. You may also want to check that you are using the most recent drivers for your soundcard and other cards.