I just have to add this - while I am all for "maxing out" boxes...

If you can put 2 GTX 590's in two "un blocked" slots, then just put your money there. Can't do more than 4 GPUs on a Windows OS anyway (at least not without a lot of tricks), and a single 590 counts as 2 GPUs. So 2 GTX 590's and there's your limit.

So, any money spent on "other" cards, no matter how big or small, is in the end, money somewhat "wasted" perhaps.
I hope that view point helps a little...