Any linux distro with Compiz Fusion (comparable to Vista's Aero), is nice to look at.
If you're a beginner to linux, I suggest downloading Ubuntu 7.10 iso image (which will come out later today), burn it onto a blank CD and run (not install) the Live CD. Get a feel of what it's like and then make the decision to whether or not install it as your main OS, or dual boot it with Windows.
Lol, just use Beryl. That thing is very fucking awesome.
What they said.
Btw, why is everyone here antilinux? Linux is much better, though not all programs run.