I've been mining with 2 x 5850 at stock speeds, each getting around 270 Mhash/s. A small speed bump to 800mhz (from 725) on the GPU gave me an extra 30 Mhash/s each. I'll try giving more voltage to the cards to push the GPU speed to 850mhz. For the longevity of the cards, I think I'll avoid running 100% 24/7 with raised voltage on them.
On another PC, I have 2 x 5830 mining at stock speeds. The increase to 850mhz (from 800) made very little difference and a bump to 900mhz caused my PC to crash. This is of the XFX variety. If your motherboard can support crossfireX, you can plug up to 4 of these in. Assuming each at 230 Mhash/s, that's 920 Mhash/s at around £300 which is very reasonable. Though one has to compensate by purchasing a good quality PSU. 5830 isn't the most power efficient out there.