I would DEFINITELY recommend you get a SmartNet account for ONE of your 501's. This will give you extended warranty coverage on the 1 unit PLUS, and this is the REAL reason that *I* got it, you get access to ALL firmware upgrades, software upgrades, IOS versions, etc.! I bought 1 SmartNet account for a PIX501 and I now have access to IOS images for our 2620 routers too!
Cheers!
RoundAbout