First off, let's get an realitycheck here. Your videocard isn't new and it was already an underachiever when it first came out. Playing Hellgate London, Bioshock or Colin mcrae is going to be difficult at lowest settings and the fact that you have only 1 GB of ram and an old processor doesn't help in getting your system any faster either.
If you happened to have a 7600 I'd think about pointing you towards some tweaking tips but with this card it's more realistic to tell you to update your pc. Anyhow, try
this topic for some basic optimisation.