Come on Marc, you should know that more RAM can in no way compensate for a slow CPU. RAM can't make a computer calculate things faster as such, more store information from the hard drive. In the vast majority of cases, 512mb is perfectly fine. When you start pumping in a long list of programs, it starts to become more beneficial, but in most systems (read: internet, Email, basic office work) 512mb is plenty.
Vista on the other hand loves RAM, and anything more than 1gb can compensate for older, slower 5,400rpm hard drives that were common-place in budget systems circa 2004 or before, making them much faster than if they had XP on.
DDR2 RAM is a no-brainer at the moment though. DDR1 and DDR3 are both too expensive to consider what may amount to an impulse buy, but if you've more than 1 DDR2 system, getting some more for you and filtering the old stuff down to a lesser used PC isn't a bad option at the moment. I've got 8gb sitting in my PC (Vista's RAM caching again :D) because it's going for pennies, and the old RAM is now in my HTPC (who's old RAM has been added to my PC at work).