The only negative to going with the 800 Mhz RAM is the price - there is a very small performance improvent at stock speeds with the 800 over the 533 (probably not real world noticeable but measurable with benchmarks). For overclocking the 800 Mhz RAM will definately give you more room to crank up the FSB.
In a nutshell (At least I'll try to do it a nutshell)...
Processor speed = CPU multiplier (9 in the case of the E6600) * the FSB (266 stock - although it is "quad-pumped" for the 1066 stated FSB that you see listed)
Memory Speed = FSB (266 @ stock * FSB:MEMORY Ratio - hope I got that in the right order * 2 for Double Rate) This means stock memory speed with a 1:1 ratio is 533.
When this ratio is out of sync - not 1:1 - there is a performance penalty but it can and usually is over-compensated for by the increased memory speed)
To OC the processer you raise the FSB if you keep the memory ratio at 1:1 and faster DDR2 will allow this. You can take the FSB to 400 (or 1600 quad pumped with the memory at 1:1 at 800 Mhz - no OC on the memory) if you have the 800 Mhz RAM.
Well, I play Oblivion, BF2, DAoC (latest expansion), and multitask quite a bit. I know my rig is nice and I wouldn't upgrade if I knew it wasn't worth it. My best friend's computer is sort of a pile. I'm gunna sell him my mobo/proc/RAM, then I'll get the C2D E6600, Gigabyte mobo, and some memory (not sure what speed to get, which is why I posted heh).
One big thing for me is my monitor. Native Res is 1280x1024, which is pretty high for a single-card playing Oblivion. Honestly it still runs great for Oblivion, but when playing DAoC (especially in the Lab.), the lag is just crazy. Any upgrade will help that, so I do have a few (though not really wonderful) reasons to upgrade.
Thanks for the response, it helps a bit. I might just go with 800 for the uber overclocking =)
I have 2 GB of DD2 533 memory and have thought about upgrading many times but every time I research it I come up with maybe a performance gain of 5-8 fps. The performance increase doesn't seem to justify spending the $300 or so to upgrade.