Everyone agree that the 1/2" fittings are the way to go?
Not necessarily. Buff and I disagree a little on this. Some of the lowest temps I've seen have been on 3/8" tubing/fittings. It depends on how much you have in your loop.
If you're only planning on cooling a CPU and don't think you'll ever want to do more than that, then 3/8" is the way to go, IMO. It's smaller and will clutter your case a lot less (not all guys have a case as big as a house like mine) while providing relatively the same performance. It's when you're running multiple blocks in your loop (CPU, chipset, graphics) that 1/2" tubing starts to come into play. And even then, I haven't really seen any major benefits from it.
I'm running 7/16" on my stuff w/ 1/2" fittings. If you think you might want to watercool more than your CPU in the future, then 7/16" would be a good compromise. It's large enough for 1/2" fittings and it's a lot smaller diameter than 1/2" tubing is so it will clutter a lot less. Here are some comparison shots:
Everything in that shot is 7/16" except for what's on my pump on the right. That's 1/2"--only because nothing else will fit it. The only thing I don't like about the 7/16" is that it starts to sag after a while. My 1/2" tubing had a thicker outer diameter that didn't sag at all and I could do some pretty cool twisting effects with. This 7/16" is starting to sag a little.
I'll redo my loop in a month or so and probably install some Tygon 1/2" when I do.