The one thing RAM wise that'll improve your speed is CAS latency on that setup, and it depends on whether the BIOS will let you set it or not.
CAS latency basically is, how many frequency cycles does it take for your RAM to transmit data. There are two grades, 2 and 3. 2 is obviously better, but you need to be able to specify 2 in your BIOS for it to work. You can't use two different speeds, so if you wanna run CAS2 then all of your memory sticks must be able to handle it.
If you can set those options in your BIOS, see if the RAM that came from Gateway can handle CAS2. If it can, get CAS2 RAM for your other 128 megs and you'll get a noticeable increase in performance.
Xayd