Incase you would like to mine litecoin, novacoin, bbqcoin or other scrypt based coin you will need more than that 4GB of RAM when using 4 GPUs.
You will also need powered risers for +3 cards so you don't burn your 24pin mobo connector with too much current through 12v lines.
With those risers comes the necessity to hang those cards from something since you can't directly plug them in to your mobo.

Club3d makes 7990 too, atleast in europe you seems to be able to get an 7990 for the price of 2x7970s. Since 7990 are supposed to be binned chips you may end up with better chips and thus lower power consumption or better overclocking. If you want to save those PCI-E slot on your mobo then 7990s might be something to consider as it's actually 2x7970s on one board. They do need 3x8pin PCI-E power thoug. With 1200W PSU you can probably run 5x7970s or 2x7990+1x7970 with some underclocking.

Building a rig now means mostly that you trust on scrypt or on the continous and rapid hike in bitcoin price. Those ASICs will start to skyrocket the BTC difficulty shortly and that might make SHA256 GPU mining unprofitable during the summer.