I would have to disagree with this, I have had 6 cards in 2 of my rigs for about a month. 5x5870 and 1 6950 clock 920/300 (900/800 6950)
your requesting 450W (6x 75W) through your PCI-E, i dont know what board you have but some will with top quality voltage regs and what not but you maybe stressing them and shortening there lifespan. for the sake of getting a power molex on the extender its not worth risking your mobo failing, I fried a gigabyte (yes i know, there crap). I also GAINED BACK 100+ m/hash when i connected a power molex'd extender to one of my rigs, which was telling me that the mobo was struggling provide 300W, never mind 450 you are drawing If its working fine, But my advice of MAX 4 is one well worth noting