I would adjust the memory speed first as it will give you the biggest boost and also does not heat up much when overclocked.
Try taking it up about 5MHz at a time until the system becomes unstable, then go back down a bit until it is stable again.
Before you adjust the core speed, I would suggest putting a fan on the heatsink if it does not already have one as the core will start to heat up a bit with higher speeds.
------------------
Speed is everything - Warp factor 9, Mr. Sulu!