Decisions, decisions ...
Argument Number 1:
Seems that the best games are released for both a console and PC ... BUT, often, for only one brand of console.
Say you do buy an Xbox now, with accessories, for about $300. And this December, a really cool game comes out ... but it's only on the Playstation 3 (and also PC).
Will you then go out and buy a $600 Playstation so you can play that game? Considering the amount of money spent to buy two consoles, would you be wishing you had built a new computer instead?
I have friends who never had a PC, but simultaneously had a Playstation 2, Xbox and Nintendo. That seems like a big waste of money to me.
Argument Number 2:
There are some games never released for PC that are great fun on a console. And other games allow you have friends over and plug in extra controllers, which you can't do with PC.
So you have to decide what kinds of games you want to play, and is PC or console better for that?
Argument Number 3:
You can rent console games are your local video store, but you can't rent PC games. As a PC gamer, this doesn't bother me because I typically play one high-quality game with depth for several months at a time (Oblivion is my current obsession). But if you just want a quick game of shoot'em up, you might as well save money and go with the Xbox.