your pc would hold it back from its full potential but you would still see a big improvement over a fx5200.
and the 7600 range from nvidia is pretty decent too
it may not be a 7900 but it can handle it self.
If you can stretch to £450 you could upgrade the 7600GS in my build to 7600GT. All depends on shipping costs etc though. Still think getting a AM2 mobo with dual core will help a lot more especially as some games now use dual core like company of heroes and FEAR. Also just in general pc use as well dual core will help.
EDIT: if you stretch to £450 then you can get a 7600GS with GDDR3 ram instead of the one I picked which only has DDR2.