i'm goin with lunotick here... i just don't see how people can say that 256MB is more than enough without knowing what you're doing with it, and to some extent how you work.
if you're gaming, or mostly just run apps singly, then definitely the faster 256MB.
if you primarily run memory hogging apps and/or tend to be working in several apps at the same time, then the 640MB could speed things up (even tho its slower) by reducing some swapping to the hd. photoshop, for example, recommends that you have five times the *free* memory as the largest images you want to work with. output from a decent slide scanner can easily be 30MB each (way more if you're scanning medium format instead of 35mm).. just a couple of those would be more than photoshop recommends... and if you're doing layout in a dtp and have an illustration graphics program open at the same time...
hmmm... i do like masterhoss's second suggestion, but i tend to multitask way too much when i'm actually working on stuff :þ and my computer definitely seems happier since i bumped upto 512