If you haven't already tried this, I would uninstall all current sound drivers and devices in the device manager one by one. Then download the latest soundriver off your motherboard's website. Unzip the driver (most are compressed). Then power down your system. Remove your soundcard. When you power up, try manually installing the new driver. I send all downloads to the "my documents" section of win 2k, but you may have another file to use. Then check the device manager section for conflicts. I've had better luck over the years using onboard sound.
It may not have anything to do with your soundcard, I tried something similar once and found out the mobo was the problem.. interferrence seemed to happend each time the processor was working, try playing some music in winamp while working your processor with some movie encoding or something, just make it work on full load while playing some music... if this is the problem a BIOS update may solve the issue
If possible I would also try that soundcard in another computer before buying a new one.
Well I had the same soundcard in my old system with my Geforce 7800 GS & different board with no issues.
Because I get the same problems with either my onboard or soundblaster I think it may well be related to interference/motherboard/graphics card - I just thought trying another soundcard may well be the cheapest option, now I think about it probably not the best move yet!