So you got it running now? Great. How's the performnace and show us some benchmarks if you don't mind. Anyways as for tightening that HSF, don't use your fingers you can't do it tight enough. You have to use a small wrench or plier and spend some time tightening that nut fasteners. Yeah I know it's a pain to install the Big Typhoo, but I think Thermaltake have the VX version now with the difference is easier to mount just like the stock fan. I don't know if you can buy the mount by itself though, have to check on that. Anyways make sure it's tight enough. I got mine to the point where the plate bended a little.
As for your 7900GT get some after-market cooler on that baby as well. I was using the Artic Cooling Silencer rev.3 for my 7800GTX and it's cools it very well and does it very quitely. It's the best after-market cooler, bad thing is it's pretty big and takes up more than 2 slots, so make sure you got some room. Now I got water cooling so my Silencer coolers have no more use. But I was very satisfied with it.
Show me some temps on your cpu too see if it's working right.