And depends on your system specifications. 2GB DDR2-677 is passable. If you have a good GFX, but a crappy CPU, then of course overclocking with a new cooler will help the bottleneck. Post your specs up!
If its a Kuma its a AM2 or AM2+ board, it supports DDR2 800 or even DDR2 1066 if its a AM2+ board. If he has 2gbs. DDR2 667 and wants to upgrade I would go ahead get 4gbs, 2X2 of 1066 or just leave the 667.