Memory Advice...

Hi everyone, just wondered if you could help me, I have a Packard Bell iPower8620, and i'm in the process of upgrading most of the components, i've already done the sound card, and going to do the GPU and PSU eventually. First though i'm going to do the memory, these are the current specs;

Intel Quad Core Q6600 2.40GHz

2 X 1GB DDR2RAM

nVidia Geforce 8600GS X2 (Running in SLI)

457GB Hard Drive

Vista 32 bit

Apparently there are 4 RAM slots on the motherboard, and i just wanted advice on how much RAM to get really, and what to get??

it can do actually 5GB if you can live with sacraficing 2GB to virtual ram from your HDD. and as for what kind i suspect (because its a dell home office pc) that its going to take ddr2-533 or 667