I have problems with my gc, so I am considering upgrading my gc to the HD7770 (or something else in that range like a GTX550). But do I need to up my PSU as well?

My rig:
i5 2500k
P67A-UD3-B3
2 x 4GB ram
1 dvd rom
1 Chronos SSD
1 WD 1TB 5400rpm hdd
4 x 12" casing fan (no brand, rm10 each, cheap ones)
CM CPU fan Model 101

Occasionally plug in 2-3 usb flash drive

Now using Asus GT9800 gc
and my PSU is CM Silent Pro 500w

Can I upgrade my gc without changing my PSU? Budget issue.
PM me price for GTX550 or HD7770 if you have. Thanks.