Having a big ol floor fan wont work well. It doesn't have the necessary pressure to squeeze in fresh air between the cooling fins. You're probably better off cobbling together 2 80mm fans and tying them on. 120mm probably won't have the necessary pressure, plus if they're anything like the non reference 5850's, that is a loooong heatsink.
If you're intent on mining with it, find some 80mm's and tie em on. If you're gaming or selling, better start looking for replacement/aftermarket parts unless you're comfortable selling as a frankenstein.
I used some zip ties and dropped a fairly decent silverstone fan level w/ the left side (haha)... I can still only get 340mhash with it (750/150) without having GPU temp go above 70o (and VRM's above 76-77o)... but at least it's going again.... 75% efficiency, not too bad..
I'm starting to look around to see where I can find one of those gimpy fans that gigabyte uses in their cards. Shouldn't be more than $10-$11, I'd think..
I could also do what you said, just tie on some other fans... like some of these replacement ref fans. I'd need to buy some 4pin extension cables or something though..