I'm thinking of upgrading my PC I currently only have a Intel Dual Core 2. 70GHZ Cache 2MB and 4GB Ram. My Video Card is ok I believe as it's HD 1GB, forgot which make.

I was thinking of getting another 4GB of Ram and replacing my Processor with Intel Core 2 Quad Q9400 2.66 GHz Cache 6MB (is the Cache size important or just the GHz?) . I'm just wondering would this processor be much of a step up or would it be better to save a little more and get a i5 ( Intel Core i5 2.66 GHz Cache 8MB or Intel Core i5 3.33 GHz Cache 4MB)?

Thanks for your help.