What operating system do you have? If you have Windows XP 32-bit you will not be able to utilize more than 3.25 gigabytes of memory. Windows XP 32-bit doesn't recognize more than 3.25 - 3.5 gigabytes of RAM.
You will likely not see much difference in performance with DDR2 667 and DDR2 800 Random Access Memory. I would go with DDR2 800 memory if you want memory for gaming.
I also would go with a good brand of memory like OCZ.