Your cpu is fine ditch the board and overclock and then upgrade if you feel the need. The 8800gts 640 is ok but obviously showing it's age but if you want nvidia a cheap gts250 or gtx260 will be world better than what you got now. If you want ATI a 5770 if you can get one will take your breath away or better a 5850.
I just saw that the 5770 needs +40A on both +12 rails .. which I don't have with my current PSU.
I used a x1900xt with a 5 year old psu with no problems plus the x1900xt has the same power draw as the 5770 at max load. You will be ok unless you already got low volts then you shouldn't even upgrade the cpu till after. Don't worry the max for this card is actually quite low.