you can mix and match 32 mb,64mb,128mb,256mb,512mb,1gb,2gb,etc... as long as the are 184 pin. For a note if you mix a pc2100 and a pc2700... the pc2700 will run at the pc2100 speeds... so in other words if you have faster bus speeds on one ram it can limit your system... as for what ram to get you'd want ddr 400 or ddr2 ram for the best quality... ram timings can also be important... the best timmings are 2-2-2-5 (these are usually listed with the specs. and you WANT CAS 2).
P.S. hope i didn't put you to sleep