Make sure you don't get four 512 MB modules - this can restrict overclocking. With two 1 GB ones, you should be fine.
Now, a few words about the performance. In most games, 1 GB is absolutely enough. 2 gigs don't really make a difference in benchmarks even in the most RAM-intensive games like F.E.A.R. or BF2. However, some loading times do improve a bit and with only 1 GB, you may experience some stuttering due to swap file access. This isn't reflected in average fps comparisons but it can be a deal breaker in some situations. Therefore, for a new system I recommend 2 GB. With 2 gigs, you're on the safe and more future proof side.