At $20 the XBee is pricey. It is extremely simple to use.
If your sensors are in close proximity you may be able
to use a single XBee to interface all of your sensors to a
single XBee.
How big are your webpages and how much data are you
planning store. Are you sure you can't store everything in
the ATmega? The new ATmega328 works in the ATmega168
socket and gives you an extra 16K of flash.
The SD card should not be a problem if you need that much
storage.
(* jcl *)