In some ways it depends on the programs that you *do* use, which if they're from Redmond tend to get bloatier with every release. Often, a pile more RAM (3gig max on older machines) can immediately give you a benefit, and it's very cheap nowadays so no excuse not to.
The point about checking what's running even when you have no apps open is a good one though - you may have any number of unnecessary processes running. Anti-Virus software can also be a big resource hog, particularly Norton (IME) - think carefully about whether you want mail to be virus-checked on the way
out, for example, and whether applicaitons are virus-checked on startup. As long as you're confident your machine is clean, that is...
Responsible for the techie bits. Has biased opinions.