Pioneer A05, XP1700+@2200+, 768MB Ram...
Not taxing at all. Doesn't use much CPU (2-3%). But when I had everything on a single IDE drive, I had to be careful not to launch anything that required significant HDD access. I did coaster a few that way. For normal surfing and mp3's it was fine. You couldn't tell the difference.
Now I have a SCSI setup. One OS drive, one app drive, and my IDE drive for data. I can play Wolfenstein and burn a disc at the same time. That rocks.
Anyway. Watch your HDD access and you'll be fine. CPU is not a factor.