My advice is based solely on budget, not practicality. I completely agree than buying a wireless powertap wheel is the right answer. That being said, once I got my power meter, I really don't care about the garmin anymore AT ALL. The powertap computer is good enough for what I do, and the gps thing, while totally cool, is less useful the more I ride. I am obsessed with power numbers, though, and I would be 'lost' without the powertap!
So, if budget is a factor, or if you are not really concerned with the GPS function, competitive cyclist has wired powertap wheels with the computer and the whole thing for $660, which is a pretty good deal. I got mine on ebay for about $350 when I was done, and a friends was $450. If you don't want the wires, look other places than competitive cyclist.
just to fill in the blanks: wired powertap won't interface with the garmin at all, but you can use the same heartrate strap. The wireless models interface with the garmin directly.
hope that helps