You need 64 bit Windows in order to run more than 3.5GB RAM, just FYI.
Otherwise, I would say check either your motherboard manual or the manual/specification sheet that came with your PC/motherboard to see what the max supported RAM is. Also, check to make sure you have free slots on the motherboard. Most standard ATX motherboards have 4 slots for RAM, unless it's one of the older Intel boards in which case it might have 6. Smaller boards like mATX and mini ITX boards usually have 2 slots, but occasionally you'll see 4.
Hope this helps.