If you want a great gaming machine, get a new PSU (per above) 5850 (along with everyone else scrambling to find one in stock)
Another option would be to get the 5770 and NO power supply. The 5770 only takes one 6 pin. It will still be significantly faster than your 8800. I replaced a 9800 with a 5770 and it is a substantial improvement (though not doubling like a 5850 would be).
Another smaller improvement is to replace the CPU with a PhenomIIx4 955. Some games that would help on and some not.