I have just one 5850. It pulls ~273 Mhash/s at stock speeds. Now, i know that solo is near impossible even with dual 5970 and will be even more so, however do you think that the chance of finding a block increases if you have multiple instances of a miner running? I have 4 currently on my 5850 and the speed is somewhat divided for every miner.
Assuming that a quad 5970 uses the same method, don't you think the probability get's increased by a small fraction that you will find a block faster than with one miner? I mean it's basically the same thing with pools. Every worker requests new works, increasing the chance of the bitcoind sending the winning hash.