I'd generally suggest buying a nice graphics card - out of personal experience.
My cpu is rather old but runs stable overclocked @ 3.6GHz (Core 2 Duo E8600 stock @ 3.2GHz).
My graphics card is a HD6870 which probably is faster than my cpu provides but I noticed that modern games are kinda stuck on the amount of cpu they use and go way heavier on graphic power. So, my suggestion basicly is: if you need to save money, don't do it on the graphics card.