if you arent using vista i see no real reason to get 4GB of ram. unless maybe you do a lot of mutlitasking/encoding and whatnot, then you might benefit from it.
XP and linux just don't use the resources to need it. Vista on the other hand, i don't recommend under 3 or 4GB for optimal performance.
and if you get g.skill check the timings first ^^ theres lots of sets with slower timings. Great ram though.