First off how much you got to spend on it ?
You are right that building your own is a lot better and cheeper too. It not that hard to do, just take your time. You will need to buy windows 7 for a product key to intall windows 7, you be looking at 64bit version so you can have 4GB ram +.
For my last desktop I reused a old case, dvd drive and hard drive. Use the hard drive you got now as a 2nd drive to store large files and crap. A new hard drive should be alot faster than the one you got now. I spent £600ish on i5 CPU, 4GBram, motherbored, PSU and graphic card. As for a new monitor is up to you. The cheep ones I got do not make my £180 graphic card look super super good. Might buy a HD monitor for it.
There some list prob a 1/2 year out of date coma posted for me when I was looking for a new build. Should get you started but Coma should know what best to buy right now for your money.