For a general deals site check out:
http://www.redflagdeals.com/
They have a very active forum with a good deal section.
For a specific PC, it kind of depends on what you are comfortable with. If you are ok with a clone
Memory Express is a good company and gives good support to what they sell. I've got 10 PC's from them in the office right now and they've been prompt and responsive to any problems I've had. Plus you get to decide which components are more important to you and put your money there.
For brand names, I don't mind Dell. If you get a $299 system then yes it's going to have cheaper components but you won't be able to put together a clone system for anywhere close to that (heck even Windows XP Home costs $150 by itself). Don't get the upgrades from Dell though unless they're free or very cheap, often their upgrades can be done for a lot less locally.
AMD vs Intel
AMD chips are fine and are usually cheaper for the same level of performance. They also run significantly cooler than comperable Pentiums. Only in very specific tasks does one noticably outperform the other.
Dell LCDs
Depends entirely on which one you get. There are only a few different manufacturers of panels so often three different models from three different brands will use the exact same panel. Some of their LCD's are TOP notch. The 2001FP is very nice (uses the same panel as the 21" Benq and Viewsonic). I have a 2005FPW that I got a GREAT deal on and couldn't be happier.
In general though I would
highly recommend buying the LCD locally from a store that accepts returns. That way you can try it out for a week and change it if you aren't comfortable (though Dell does allow returns shipping time can cut into your evaluation period). There are some good review sites that do actual quantifiable tests as well; trust those rather than a person's subjective opinion.
For used stuff, the bargain finder has worked well for me in the past, and the forums in the site I posted above has a very active buy/sell community as well.
I agree with Wolfwood, a 2.5GHz+ processor will do you just fine. 512MB RAM minimun. A nice large hard drive with 8MB cache will keep things hopping, and a decent video card (ATi 9600 Pro) so that you can play some games if you want. Add in a DVD burner and you're set.
It's all about the budget though. If you want to spend $300-$400, one of those Dell deals is very hard to beat. Be prepared to spend extra on upgrades later though (DVD burner for example, video card if you ever wanted to play games, and some of them aren't able to upgrade the video card).
If you want to spend $700-800+, then a clone system will do the job nicely.