For what you are looking for, a Dell would be perfect. There is no point to switch to Mac unless you are going to use OS X (Jaguar) In my opinion, if you are getting a Mac to use windows it's just a waste of money.
I currently run vista premium on my laptop. I really like it, it does have some minor bad things but when SP1 comes out (should fairly soon) a bunch of things will be tweaked and fixed. If you are looking to have the fastest possible computer speed, get XP with 2gb of RAM. Vista is still really quick (i'm running it with 2gb), but not quite as quick as XP.
No matter what, I'd make sure it has 2gb of RAM right away. Chances are if you buy one with only 1gb, that they will come in 2 sticks of 512mb. Guess what that means? When you want to upgrade to 2gb (or more), you have to buy 2 brand new sticks of RAM, and pretty much toss out the other ones. Also check into a Dual core processor.
As a company, i've been pretty impressed with Dell over the last couple years. I would suggest visiting their website and building your own computer. If you visit the small business section you can get basically the same computers that are in the Home section, but for less.
Sorry for the novel, I hope some of this made sense and possibly helped.