The overall speed of the RAM is 400MHz, but in the BIOS it'd be listed as 200 because of 200x2 = 400 due to DOUBLE data rate (DDR)
You can run RAM that's DDR500, but it won't simply plug and play like that. You'll have to manuall go in there and adjust your CPU FSB or HTT if you have an AMD64 to get up to that speed the RAM is capable of, otherwise when you first plug it in, it'll simply underclock.
You don't need high performance RAM like Corsair XMS unless you plan on overclocking it, if you aren't going to OC valueRAM will save you cash and not hurt performance.
OCZ Platinum Rev2
All those IMO are better than corsair in terms of overclocking, but of course they also come with a pretty penny price tag