Taking the Bottom Up approach lemme answer what I can.
Corsair is a good company whose every product is of guaranteed good value. You can always tell the quality of a PSU by holding it in one hand. heavier the better albeit if the manufacturers haven't deliberately put rocks and junk in it

. Corsair's PSU is costly 'cause they don't cut corners in quality of product, their capacitors are heavy duty , insulation is of good quality and Fan's sturdy too.
As far as cooling goes if ur not updating ur cabinet and don't have
an AC in the room where ur system(Passive cooling also matters) is then
I suggest u get at least 2 90mm fans installed in ur cabinet and since ur going to invest in a 8800GT card it won't hurt to get a decent 550- 650W PSU as you know these cards require more power and their successor like 9800GT, just don't ask. Moreover it never hurts to be on a safe side. BTW my mobo is also a 945G chipset based and at times proves to be a bottleneck for my 7600GT, so its gonna definitely be an ass-plugger for ur 8800GT