PCs work fine, I use one with Cubase and it has served me well for almost 4 years now. While I will admit that Windows comes loaded with **** by default, a quick Google search will enable you to pare down your OS to a bare-bones level and dedicate the computer's resources to important stuff like incoming audio data.
Outside of Mac VS. PC, get the fastest CPU you can get your hands on and the most RAM you can, as well as a ton of HDD space. All the .WAV files in various recordings add up quickly. I have 800GB of HDD space that I've filled about 60% of in just under 4 years, to give you an idea. More RAM & CPU will enable your computer to handle more tracks, effects, automations, etc all on the fly without choking and introducing pops, clicks, bird chirps, etc.
Also, avoid USB for audio signals. Either use FireWire (with a GOOD QUALITY Texas Instrument FireWire chip-set on your motherboard) or use analog cables with a dedicated sound card. M-Audio makes some good entry level sound cards that will allow you to plug in XLR, MIDI, etc. They have good low latency and support for 24-bit/96kHz recordings.
I've only used GarageBand a handful of times and I hated it. It just seemed like a toy to me. But I guess the fact that I come from Cubase doesn't help that. If you're just starting out, perhaps it could be beneficial. Personally, I just dove straight into Cubase when I started and fought my way uphill. It does have a very steep learning curve. Even now, several years after starting with it, I still learn stuff about it. It's a great program, very, very well made, very powerful, intuitive and extremely stable.
The SM57 is a great mic choice. Inexpensive and versatile. It's not ideal for vocals, but on a budget, it will do just fine. You might want to look at a brand called Motu, they make some fuckin' spiffy pre-amp/FireWire/analog units with built-in effects. Something like this: http://www.motu.com/products/motuaudio/ultralite-mk3/ My buddy has one of those, it's a great unit.