Buying more RAM [duplicate]
If your motherboard supports (and most likely it does) dual channel RAM you should buy 2x1 GB modules.
As for buying 800 MHz - if you combine different speed modules they will operate at the speed of slowest ones. So if you don't intend to change old RAM but only add 2 GB, buy 667 MHz if you get it for cheaper.
go to www.crucial.com and use their Memory Advisor tool. select your mainboard manufacturer and model and you will get a recommendation which modules suit best.