Those 285GTX's I was looking at can take 185Watts on the low and 600Watts each on the high with 3 seperate 6 pin connectors so you can overclock the shit outta them. Your 295's can prob. handle quite a bit as well.
I was running my 8800GT without it's seperate power cable attached and your right it runs but at half mast if you don't get that seperate power on it's own rail to it.