rolli59 is right, with one video card 800W is overkill. You can also save some money on your case if you wanted too, although the one you chose is nice. You're a little under your budget as it is and downgrading the psu and/or the case will put you even farther under. I would use that savings on a 120GB+ SSD for your operating system and games which will run you about $200 but will certainly make a difference when optimized properly. A cheaper option for increased hard drive speed is buying a second identical Samsung HDD and put them in RAID0, but the SSD is more reliable. These upgrades are luxury items, though, for increased performance - if you don't want to splurge, a 7200RPM should suffice.
Yeah guys he said "probably not" for sli or crossfire. And also, just to throw in my 2 cents, the 2500k benefits most from ram that is @ 1600 mhz. Anything above that you will not notice a performance increase, but anything below, you may notice a decrease.