Hi Iru,
I would suggest before you spend any money that you first spend some time with a profitability calculator (or two, some are better than others) to see if it is even possible to get your money back with the expected difficulty increases. If you are not getting this stuff for next to free and you don't live somewhere with cheap power I doubt you will ever make your money back in Bitcoin with GPUs. If you are looking to mine some alt coin you may have a chance to make some money if you happen to choose one that takes off but that is a gamble in itself. 4 GPUs in a single case generate a ton of heat and unless they are water cooled the middle cards will always be heat cycling. I had 3x6970 MSI lightnings over clocked and had lots of heat problems with the middle card. Not to mention the box pulled 1kW at the plug while mining. I shut that miner down at the end of last summer because it could no longer earn enough to pay for its own power bill.
There is a lot of good info at
https://en.bitcoin.it/wiki/Mining_hardware_comparison to see what you can expect for hash rate from different cards, although I have found the reported hash rates on there to be on the high side.
Good luck.
