Not sure if the 2360QM can take 1866MHz RAM, think I remember seeing that that one couldn't take the faster RAM, I'm sure someone else will post as to whether it can or not. The 29020XM definitely can, so if you're buying one of them it won;t be an issue.
As for the GPUs, you can take out the 460M's and put in SLI 580M (you should already have the SLI cable with your 460M's but you'll probably get one with an SLI 580M upgrade kit. If you go 6990M CF you'll gat the crossfire cable in that kit too I would expect, unless you buy the cards separately on eBay. They will be pretty hard to find I'd expect; you can buy them from a Clevo reseller in the US but I think they may have to be flashed with the Dell BIOS for them to work in SLI/CF in the M18X. See here:
The CPU is really up to you ES (Engineering Sample) or QS (Quality Sample). There are rumours that some ES products aren't as polished as the QS or the OEM but I see a few people on the forums here that have them. Just make sure that the person you buy from has a very high feedback score and a large number of sales (even if slightly more expensive) as you're less likely to get burnt.
thanks everyone! i will be buying the ES 2920xm for around 290, how does overclocking work on the m18x? is there total voltage control for everything on the mobo? im coming from being a desktop overclocker