I agree completely with sturm, I have wittnessed it first hand on many mobo's. 2 x 512MB DDR400 DIMMS run perfectly well in dual channel mode, then another identical 2 x 512MB DDR400 DIMMS are added and the system drops the memory speed to 333MHz and I have also been told by a mobo maker, no names (a bit of time ago) that putting 4 unmatched DIMMS on a board could cause system instability...Not good.
Also I would not advise mixing sizes, i.e. 2 x 512MB with 2 x 1024MB, this is not recommended by any mobo maker I've had dealings with, they all advise matched DIMMS all the way.
2GB will be more than enough for any task you can throw at your machine until VISTA arrives, and then you may think about adding another 2 x 1024MB DIMMS (Matched to the first set if poss)
I hope this helps
JKay6969