I would suggest using 2 identical RAM modules for dual channel (so not 3GB). Dual channel doubles memory bandwidth, and is supposed to increase performance, although I don't know how significant.

So yeah, 2x2GB or 2x1GB. Since RAM costs like nothing these days, I'd say 2x2GB.

Don't listen to those enthusiasts who would tell you to get 8GB of RAM or some bullshit like that, even 4 is probably more than you'd be able to use up.