If your going to buy a pc don't buy from Dell, or from Hp.
Go online or to your nearest computer specialist store and order a custom computer that has features to fulfill your particular needs.
E.g. If you play alot of video games, buy alot of RAM and a good graphics card.
If you use your Pc as a multimedia system make sure you get alot of hard disk space in order to save your movies, tv shows, music, pictures etc.
It depends how serious you are, but a custom made computer is the way to go because they are generally cheaper, have longer "money back guarantee's" and are put together by hand not by machines in a factory.
For convenience and cheapness i would buy online and have it delivered to your door, but i know some people don't like this so just drop into your nearest computer specialist store and tell them what you will use your computer for and they will put it together for you.