Or consider using DVD media to store all your music and video.
Name brand computer manufacturers tend to make their components proprietary so you simply cannot upgrade. They do this in hopes of compelling you to buy a new computer instead of adding to the one you have. Compaq and HP (now one company) are notorious for this, although Dell and Gateway do some of it, too.
As our friend Xandax recommends, you want to start by setting a price you are willing to pay and then buy the best computer you can put together for that price.
If you are looking for a low-end machine that will handle internet surfing, low-end games, word processing and simple multi-media activities, you are better off buying one from Dell and adding some extras (ie-a second HDD and more RAM). You can often find these in the $600-900 range (no monitor included). Plus, they also often have a warranty that is longer than a shop's.
If you are looking for a mid-range or high-end computer, you are much better off either building it yourself or contracting a shop to build it for you. However, if you are going to have a tech or a shop build it for you, you had better trust them or you are going to be in a world of hurt if/when it breaks down. I've also found the website
http://www.cyberpowerpc.com has some good prices, although I do not have any experience with their service.
One other thing to consider is this: are you also going to be upgrading your other compoents, or will you be using your existing peripherals (monitor, keyboard, mouse, etc.)? If you are going to keep everything except what's in the case, you should have one built for your.
You might also consider keeping some of your other existing hardware. A HDD, your optical drives, network card and some other things could be recycled into a replacement machine.
A good shop wil move your current hardware into a new computer for you.
If you are going to go with an unbranded computer, be aware that you will either have to assume some of your own technical support or you will have to rely on support that is not 24/7 like Dell's. fable makes this point quite well. However, you are apt to get more value for your dollar if you have a machine custom-built for you.