Your mobos max mem support is 2GB - youyr mobo has 2 RAM slots and it supports 1GB per slot - that makes 1+1=2GB in total - looks like your HW expert bro has made a mistake in here
I don't think your mobo suports ram OCing and laggy performance may be due to the absence of a gfx card in your system - get a gfx card, install the latest update patches for the game and lags will be gone from GRAW2
1 GB DDR2 800 Mhz ( 533 Mhz and 667 Mhz modules are very hard to find - so can't provide any price info of those ) ram module price is around ~0.85K - though make sure your mobo has the latest bios version.
BTW, your mobo has some issue with High speed rams and before upgrading ram I would suggest you read these :
Compatible modules for Intel D102GGC2 mobo? - The Corsair Support Forums
Support for the Intel® Desktop Board D102GGC2