Just an idea: get yourself an EVGA Classified Super Record 2, it has 6 pci x16 slots and i think is priced between 450 ~ 500 quid. Then as you can afford it (as the board isn't exactly cheap) bung in a 460 superoverclock, and gradually add to your gpu array. couple of low end i7s and 6x 2GB ddr3 sticks to split between them in triple channel. You'll need a beefy psu too but it will be overkill until you accumulate 4> cards. Again, it depends on your budget but will be a nice project to add to each pay day.
Or, you could get couple 580s on a more standard board..