Incompatible RAM can decrease the speed of your machine.
If BIOS recognises the RAM then it should be ok - the manufacturer of the RAM is irrelevant as long as its the same
type - DIMM SIMM etc
clock speed - 333mhz etc
voltage - 2.7 etc
if all those are correct it should work fine
But like achenar says above - 3gb RAM is quite excessive and very few games will actuially shunt that much data into RAM - so you will still have HD accessing going into an Instance in
WoW for example
I doubt theres much incresed efficiency between 1gb and 3gb =)