your system will severely hold any modern graphics performance..
well, opteron 180 is an old cpu, on par with athlon x2 4800+ and struggle hard againts E6300 which is bottom end of first core2duo generation..
But i heard this opteron is easily overclockable, you can gain noticeable performance at 3Ghz..
Okay, HD 7750 maybe the strongest card for your system..
yes, it'll be slightly bottlenecked, but even old 9800GT will be..