I'm on my third Dell (in 12 years or so), and have never had any trouble upgrading them. I've doubled the memory in the one I have now and replaced the hard drive. In fact, the only negative thing I'd say about Dell is that their telephone service is now in India, and I have trouble understanding Indian English.

When you go to their website and find the basic machine you're interested in, click "customize it". That will let you add and subtract things like memory, CD/DVD drives and monitors. You can save a (very) little if you select "no monitor" if you think the one you have is ok. They also have a link to their "scratch-and-dent" store, where you can get stuff that has been returned and refurbished. I got a 17" flat-screen monitor there which was well worth the price I paid.

Different people have had different experiences with HP. My daughter had one and, the second time she tried to get a major component replaced under warranty, was told that replacement was a one-shot deal.