Nobody can really tell you how much it will help on your computer there are to many things involved like defragmentation, hd space and speed, cpu speed etc..

One tip if you ever going to buy (build) a new computer visit sites like http://tweakers.net/reviews/702/4 (dutch site)

I bought all my computer stuff online and configured everything myself most of the time it's cheaper than buying a (standard) computer in the shop cause you can left out allot of unnecessary stuff

and your computer will last longer because it will meet the recommendations better for newer games