Computer purpose : Medium-High End Gaming
Parts excluding: I already have a monitor and may not be purchasing a new one for this rig.
Overclocking: Some but not a focus
Purchase Date: Summer 2013
SLI/Crossfire: I would prefer to start out with a single card with the ability to add an additional card a year or so later. I am open to SLI/Cross from the start if there is a good reason to.
Parts list so far:
Antec Nine Hundred Black Steel ATX Mid Tower Computer Case with Upgraded USB 3.0
ASUS P8Z77-V LE PLUS LGA 1155 Intel Z77 HDMI SATA 6Gb/s USB 3.0 ATX Intel Motherboard
Since you're going to buy this rig in summer i would suggest you to wait till then on deciding. I agree that PSU will be decided according to GPU you buy but prices may come down, and final 7990 radeon may finally come out so you'll have options to decide.
Either way, you'll need at least 1000W PSU so prepare to unload 160+ $ if you really wan't to play everything on dual screens and have options for SLI in future.
But that all should be decided when time for buy comes.