On power supplies - there's almost nothing sensible a decent 500W can't power these days. PSUs above that are a mixture of 'marketting' and 'mentalists running mad overclocks and 3 GPUs and liquid cooling and shit'.

Power Supplies do have a finite life tho - even the best PSUs will lose 20-25% of their effective power over 3-5 years (crap ones lose 20% a year) which means even good PSUs are less than ideal as they approach the 4/5 year old mark (I've yet to see an PSU pass 6 without issues).

To the OP - I'd say that a new GPU will be useful when you decide to upgrade everything else anyway - that 5770 would be sub-par for a nice shiny-new i5+mobo tho so perhaps that's where you should start - you'll see some benefits for sure (and I'll have your 5770 :) ) :)