New to DDR2

[Martyuk39]

Building a cheap system for someone - thought I'd try an AM2-based system for the first time. So need DDR2. Usually buy from Crucial - 4200, 5300, 6400 - I understand each type has different characteristics and that presumably if I spend more money I get a faster machine. But if speed/overclocking isn't that much of an issue, and stability is, then which flavour should I go for?
Thanks
Martin