Reasons to Go Slow
If dependability is your goal, stick with straight 802.11b. I don't recommend purchasing multimode NICs yet. Many vendors now ship 802.11b/a/g cards, which sound like a perfectly delightful way to cover all the bases, and to be ready for when there is reliable Linux support for the a and g protocols. Given the sad history of Linux support (insert ritual snort of derision), though, I would wait for a sure thing.
Also, a new standard to replace the now-elderly PCI bus is emerging: ExpressCard. At the moment, it promises to be built on platform-independent standards, be very fast, and cost less to implement. Look for actual products to appear in 2004.
Yes, it's a ways off, so don't hold your breath, but given the reluctance of manufacturers to support Linux on existing wireless products, especially on the higher-speed devices, it may get here first anyway. If it lives up to its promise, writing drivers for different operating systems will be trivial, or possibly even unnecessary.
For Those Who Must Have Speeeed
Far be it for me to stand in the way of those who must live on the bleeding edge. For the fearless speed-demons there are a couple of 802.11a/b/g-on-Linux projects for Atheros and PRISM GT chipsets (see Resources). Atheros dares to support Linux well and publicly — bless them.
Wireless Access Points
With the exception of higher cost, there's no downside to using multi-mode access points. WAPs come with their own embedded operating systems, so OS compatibility is not as much of an issue. But there are still some irritating deficiencies. For example, many access points come with nice management and monitoring utilities that only run on Windows, and units with upgradeable firmware often require Windows to perform the upgrade. Still, although they have their limitations, these APs are usable on Linux.
Look for WAPs that can be configured via a browser interface; these will work anywhere. These APs typically include network settings, user management, WEP (Wired Equivalent Privacy), and support for roaming. Other common options are routing, bridging, DHCP server capabilities, and the ability to function as an Internet gateway.
The downside to units with browser interfaces, especially the lower-cost ones, is you're stuck with them — if you want to ssh in directly and perform cool command-line tricks, too bad; you can't.
Some other things to look for in a WAP: