If $500 is your max then get an
8800GT for sure, you'll be very underbudget, but highly satisfied for $240-$250. If you want lower, go for an
HD3850, or even cheaper maybe an
X1950PRO.
Any of those cards will put that onboard POS to absolute shame. It's like comparing a tricycle to a 911 Turbo...
Aside from Crysis, an X1950PRO will probably have you maxing out, or very close to maxing out most games at medium resolutions.