|"slowly turning this into a non-oem system"|
As long as it has a Gateway motherboard, it's an OEM system. The crippled Gateway BIOS is what prevents you from tweaking the settings & getting the most performance out of the system.
"if i were to buy two more 2gb sticks of ddr3-1066 and add them to what i currently have would i have any trouble since the four wouldn't be matched"
Match them up as closely as possible. Same speed, same voltage, same latency (CL). And make sure they're paired up correctly for dual channel mode, in other words, old w/old + new w/new, not old w/new + old w/new.