2.1 is back compatible to 2.0
If you put a 2.1 in a 2.0 slot, it will shrink down to a 2.0, you won't lose much, don't fret.
Linksys is BS. Well, their saying that it only works in one computer, and their products, unless you get the high grade stuff, is pitiful.
You can use a wireless card in infinity computers until it dies or drivers are no longer supported for a said OS.
I would personally look at D-Link or Netgear. Or even an off-brand, but make sure it has a Realtek chip on it, you can see in picture on Newegg. Realtek is awesome!