But how much is enough? Some say 4GB is the magic number these days, but it really depends on a variety of factors: how many programs you run simultaneously, what kinds of programs they are, and so on.
Over at Lifehacker, the How-To Geek answers common questions about RAM. Specifically, he explains what extra memory will and won't do for your PC, identifies which programs can actually take advantage of 4GB or more, and clarifies some of the 32-bit-versus-64-bit confusion.
It's one of the best articles I've ever read on the subject, so if you're thinking about upgrading your PC or buying a new one, it's required reading.
I can attest to the benefits of lots of RAM. The system I purchased last year came with 6GB, and it runs smoothly whether I'm using two programs or 20. Of course, most of the time I have maybe half a dozen apps running, none of them Photoshop, so 6GB is probably overkill for me. But better to have too much than too little.