Go with an AirPort card. They're not expensive (around $70 on eBay and other places), the machine and card are MADE for each other, and because of that, they work together wonderfully.
Ok, there are some downsides: the basic AirPort card is 802.11b, with a max data rate of 11Mbps. For a desktop that's not a bad thing, because it's not that big a deal to use a cable to connect for big file transfers, and surfing with a "B" card has never been noticably slower for me than with a "G" card or a cable. Laptop users obviously don't like the need to connect with a cable for big file transfers, but that's a tradeoff based on what generation of laptop they have.
Remember, when searching for an original AirPort card that it looks like a PC card, while the AirPort Express card is a mini-PCI card (it looks more like RAM for a laptop than anything else).