I have 4GB RAM in my system, came from 2GB a while back.
It won't necessarily be faster (at least in terms of processing), but if your system doesn't have to use paging file, it can gain a lot of responsiveness by turning it off.
I'd personally get 4GB, just because RAM is so cheap now. and then just decide whether to get DDR2-800 or DDR2-1066
You probably won't benifit all that much from DDR2-1066 unless you're using an AMD K8/K10 system (s775 systems tend to rely on cache more than memory bandwidth)
would u still make that decision if u were me and already have 2 gigs of ram? ( comp spec details in sig )