RAM is cheap, get more RAM. Also, use alternatives to commercial programs whenever possible. You'll almost always come out better in terms of resource usage, and freeware (open source apps especially), don't ninja their way into your startup (ie. realsched, qttask, ituneshelper). Programs like Rambooster can help, but they're of less use in XP than they were in 98/ME from my experience. You can do other stuff like using a solid color for your wallpaper and switching back to the classic interface (IIRC) to gain a few more MB, but they won't be nearly as affective as cleaning your starting processes up and shying away from commercialware.
Also, get more RAM.