A more thorough answer:

I posted similar info on the Live Sound tribe but I'll be a bit more specific with regards to setting up a digital audio workstation (DAW)

1) PC or Mac? for a personal DAW it doesn't really matter. The only way it would matter is if you'd be going out and working in professional studios where most likely it'll be a Mac. But I'm assuming this would be for personal use, so if you're already running on a PC, then stick with PC. Macs crash. PCs crash. But with the release of Windows XP SP2, crashes are now few and far between.

2) PC Power? For music Almost any new computer today running at 1.5ghz with 512gb of ram will handle pretty much anything a beginner can throw at it. The video card isn't a factor in music. The only reason you would need more power for music production is if you were doing multi-track recording. Of course, a faster processor and more ram will increase performance. Since you mentioned gaming as well, that will require more power, but for music it's not nearly as important. Anyway, if you're going the desktop route, 3ghz processor, 1Gb ram, and a 256mb video card will serve you quite well for some time. If you can afford 2Gb of ram, do it.

3) What Computer? IMO, don't build your own computer. You can save some money, but not a significant amount, unless you're buying bargain parts. But more importantly, Music/Audio can be very finicky and not all hardware components get along. For example, there have been issues with the Intel 915/925 chipsets and high powered PCI-E video cards. www.adkproaudio.com/PCI-E.cfm Also, a few years ago I built a DAW, and the VIA chipset on the motherboard regularly caused glitches in recordings I made.

Dell is great in that they're inexpensive. Check out www.slickdeals.net and www.deals2buy.com/ for regular deals. Almost every couple of weeks there's a really amazing deal.

But if you're main focus is music, a boutique PC is the best route to go as they can set up a complete PC with all necessary software and hardware and some will even support the entire package. The big advantage is you can be confident it'll work. A few DAW builders

www.adkproaudio.com
www.shop-sonica.com
www.pcaudiolabs.com

4) Music Software: First, most software is available as a demo. They're typically full versions with saving disabled. So you can try before you buy. I'd recommend the following:

Ableton Live! www.ableton.com
Reason 3.0 www.propellerheads.se/
FL Studio www.flstudio.com

IMO, the easiest to learn and most fun to play with is Ableton Live! It comes with a thorough tutorial which is incredibly helpful. Also, you can use your qwerty keyboard to play music, but you're best to get a midi controller.

5) Midi Controller: it's like a regular keyboard, only without sound. It simply sends signals to the software program which in turn produces the sound via whatever synth, sampler, or drum machine you have enabled. The key is getting a controller with assignable knobs and faders so you can tweak parameters. This will give you more control and ultimately your music will be more alive and less repetitive. Anyway, here are some good controllers

Edirol PCR-30
M-Audio Radium 49
M-Audio Oxygen 8

www.pcaudiolabs.com/products.asp

6) Audio I/O (soundcard): This one's a bit tricky because it really depends on what you're doing. If just looping and such, you don't need any inputs and so you can just use the onboard audio to start. While the sound won't be as clean as a dedicated card, when you're just learning it doesn't matter.

But if you want to record audio, say an acoustic guitar or vocals, then you're going to need a mic and preamp and an audio card to accept the signal. Some audio interfaces have the preamp as part of the breakout box, so you can just plug the mic in and go.

There are a whole slew of audio cards and USB/Firewire interfaces. Again, it's best to figure out what you want to do, and then get the card that will meet those needs.

Audio Interfaces:
www.pcaudiolabs.com/products.asp

Ok, that's a lot of info, but enough to get you started.