I had the exact same problem. My Mac Pro is upstairs far from the router, and both my Mac Pro and my Dell before it had low wi-fi bandwidth. Big downloads would stall and I had to stop and restart the wi-fi adapter (Airport for the Mac Pro) to get it to continue. I bought a Hawking HWUN1 300-N USB adapter (very small with two vertical antennas for N networks), which uses Ralink wi-fi hardware. It moved my Dell's wireless from 1 bar to 4 (out of 5 on Windows 7) and everything worked great. So for laughs I checked the Ralink website, and they had a Mac driver from Sep 2010 (Hawking's Mac driver was ancient) which worked beautifully. I now have over 90% connectivity and downloads fly and web pages snap onto the screen. Works for Windows 7 in Parallels, too.
All is not roses of course - you have to have the driver load at startup, and u have to reload it after the Mac wakes from sleep (so you have to keep it in the dock). But it's worth it to solve this annoying problem.
Unfortunately the Hawking HWUN1 is no longer made, but I'm sure there are other USB adapters you could use. Hawking makes one that looks like a dish antenna that you aim towards your router (also very small like my HWUN1) which also uses the Ralink driver.