If you’re interested in overclocking, then you’re better off with 2x 2GB. Traditionally, addressing that many banks uses a trade off by using a slower command rate (1T vs 2T) and/or slower memory timings. And of course, there’s the limited amount of expansion.
Ideally, you’ll want the same RAM across the board, but it’s not required for overclocking. However, given the dual channel memory feature on many motherbords these days, it’s pretty much required to have the same RAM in both channels.
Realistically, you’re probably not going to notice the difference, but given that the price is pretty much the same, the lower timings are a plus.
Secondly, what OS are you going to run? Windows XP isn’t going to make full use of 4GB, and for most people you’re going to run into dimishing returns after 2GB at this point in time. Even for a gaming system, you’re just not likely to see much, if anything at all, going past 2GB.
linux uses all ram before if writes to the page file right? if thats ture will lunix use all 4gb first becuase it it will then for many games you will not even use the page file