You could play all the new games? You must be kidding! My 5 year old PC cannot even play Assassin's Creed(1), NFS Undercover(And the games which came afterwards in the series.), Call Of Duty : 4(That to at lowest settings and still lagging) etc. It celebrated the release of Far Cry when it was released(They gave a free copy with every system) and now it can't even play Far Cry 2!(At ANY settings). And when I bought it, it was a gaming PC!

Now I can't even upgrade my graphic's card, as my motherboard doesn't support any new graphic card, and now Windows broke on that and its running Ubuntu at a very satisfactory speed(
Unity sucks, Classic Gnome ftw!
If you can play the games you would have upgraded your graphics, and maybe the processor too!
If you still like your CPU box, I would suggest you to buy a new Motherboard. It would come with new processor(Look for Core i3+ or AMD Phenom II or Athlon II+), embedded graphics(They are not much powerful, so maybe in future you would need to buy new graphic card) and sound card as well! You could use your existing HDD or buy some new one(They have become so cheap now!) You could ask somebody who have experience with assembling to assemble it for you, it won't be difficult as you just need to put a Motherboard and HDDs!

Else you can buy a whole new PC, which would be more costly and then your old PC will(have) become obsolete!