Toma's right. If you're gonna make the investment in air tools, you need to buy a GOOD compressor. Standard 1/2" impact wrenches don't take that much air. But you'll want an air rachet too. They take quite a bit. Cutoff wheels take TONS. You'll quickly want tools that your compressor can't handle for more than about 2 seconds without kicking the compressor on to fill the tank. Once you hit that point, you're going to cycle it on and off so quickly that it'll burn out.

So, that said. Go to lowes or home depot or harbor freight. Most air tools have a rating for how much air they need (CFM). Then look at compressors and see what they can sustain.

I've got a 6hp compressor w/ a 25 gallon tank that sustains 6.2cfm @80psi. When I'm using the impact wrench heavily the compressor comes on quite a bit, but not so much that I'm concerned about burning it up. I wish I had gone bigger.