Dell is horrible because of how they custom tweak the voltage on their motherboards making you have to buy crap from them to upgrade. Why spend 50 bucks on a good powersupply on your own when DUDE you're spending 200 on one from Dell!

If you want to go with someone who will build it for you check out ABS. They're not as expensive as Alienware or Voodoo, and make very decent gaming rigs.

http://www.abspc.com/

If price is an issue, for CPU I'd get an Intel Pentium 4 3.0 GHz with Hyperthreading and motherboard combo. Check out: http://www.pricewatch.com You can pick up a combo board for around 200 bucks.

If you want to get something truly highend, I agree with Mal about going Athlon 64.

I'm not a fan of the shuttles simply because of pricing and upgradability. If you dont' have a lot of extra cards in your machine, they're a good bet, but you can typically only house one hard drive and one optical drive in it.

For GPU, Radeon 9800 Pro all the way baby. They are just a bit over 200 bucks, and kick some serious ass.

RAM, I like Corsair RAM, they make good stuff, and have never lost a stick of it. DDR dual channel is the way to go for some nice fast RAM, and you can get it for pretty cheap. I recomend a gig of it for optimal gaming goodness.

OS: Windows XP Pro. That's a no brainer IMHO

Mouse: Go for an Intellimouse Optical from Microsoft or a decent one from Logitec. Both make great mice. I also recomend a wired mouse, there's latency issues with wireless, not to mention your batteries can die out mid match!

Powersupply: 400 watts or up is good for the box, and allows for more upgradability in the future. Just make sure you get one that matches your case style. You can find some powersupplies that are cheap, but if you have a midtower the cheap supply is probably for a shuttle or a micro-ATX case, meaning shorter power cables. (I got screwed like that on a PS that I bought a few months back.)

I hope that helps. Looks like an awful lot to read through!

-C