Well your options are:
1) Get a nice screen, and hide the computer (a "low profile", i.e. small, case can help with this.
2) Get an all-in-one, just about any will look pretty good. The downside is cost and that it's not easy to upgrade. But that's always a tradeoff for design in computers, the most common way to upgrade a mac, for example, is to buy a new one.
3) Get a case with the design you like. You say you like "macs" but aren't specific about which ones. But you can try to find a case that looks like what you want. You can get the brushed steel look, you can get a mac mini imitation case, you can even get a case that looks like a home theater component. You'll have to look long and hard for them, you can start at sites like newegg.com and tigerdirect.com
4) Just get a small computer that looks ok, the HP slimline (might have it wrong but it's their small computers) is a bit bigger than a shoebox and looks decent.
5) Go nuts and build something beautiful. It's pretty easy to build the computer into a desk, or into an aquarium, or into anything you want. Here's an example you might like: http://www.bit-tech.net/modding/2006/04/06/japanese_case_mod_sangaku/1
Out of all those options, I recommend number 1. A beautiful screen is 90% of the good looks with all these setups.