Depends on how tight your budget is then I guess. Personally I'd get the 800 but you should be able to overclock good 667 to 800 speeds without to much of a problem.
What ram are you looking at? I know I had a review a while back of some G.Skill were there compare the D9 Micron 667 vs the 800 and the end overclock results were really close. I'll see if I can find it for you.
Here are a few Game Charts I found for you with some different ram speeds and timings and what they got frames per second wise. It appears there isn't a huge difference with the ingame performance. But it is a little bit better. Looking at the Half life 2 vs quake 4 Half Life shows alot better results with the different ram. So maybe it depends some on how the game makes use of it.
im not intending to max out perofrmance on high end stuff or anything. one of the first things i wanna do is get more ram to be able to disable page file because my hard drive is ALWAYS on load. i get frequent system freezes. i disabled indexing which helped a bit.
i also want a 7600gt cuz its decent and by getting a dedicated card, i can use the ram that my integrated occupies.