well unless u are running x64 O/S then you will not need more than 4GB RAM , but not sure how win7 uses ram.

you want to go in pairs due to the dual channel ... so if your ram is 800mhz speed (dual channel) then if you do not have them in pairs it only will be running at half or something.

as it is best to get the same brand of ram , i would say check the mobo and see what the max speed it can hold , get some new RAM sticks . 4GB(2x2GB) ddr2 800mhz , goes for about 75/80 AUD.