I won't get into the debate on which system is best...because there are quite frankly so many PC options available it's an apples to oranges comparison.
You have 2 choices of operating systems. Windows XP/Vista or MAC. Both are good, Vista has it's drawbacks, as you have to pick a version. MAC is well proven and quite stable without as many security updates needed.
The one thing to look for in a system now is dual core processing.
Since both PC's and Mac's now run on the intel chipsets there is some consistency happening in the software being created. You'll see it described as Binary for Windows or Universal for MAC. What it means is software that will run much faster on the new dual core machines. Right now if you have a dual core the older applications run slower than they should. Still fast but you notice it on graphic heavy apps like Photoshop, Lightroom, Aperture or CAD programs.
Buy a dual core system. All the software will be dual core for either operating system very soon. (within the next 8 months) so don't make your system prehistoric right out of the box.
Like others have said..get a system that is right for your needs. If you're not a gamer (by that they mean Doom, Warcraft, Flight Simulator,etc, heavy graphically intensive programs) then you don't need screaming fast video and monitors.
If all you do is web, mail, spider solitaire, update your website etc..then a standard machine will do you quite well.
The option I would add to any machine is another gig of RAM to bring the total to 2 gig to be on the safe side. All computers like RAM. The more you have the better your machine runs.
Have fun...and don't be afraid to ask more questions. I'm not a guru but there are some pretty sharp computer types here who will help you in your selection..just give them the specifics and they'll narrow down the choices pretty quickly.