Since I got my new Mac I have pondered Time Machine. I was thinking about getting a Time Capsule, but my router is in good condition. I am looking into an External Hard Drive that I can use for two Mac's to use Time Machine. One is my MacBook and the other is a 20" iMac Aluminum. Is there one that could plug into my wireless router and back up that way? Your thought and suggestions are appreciated. Also, how many gigs is enough? 500?