If you are measuring a few sensors and doing a simple web server I would be surprised
if a single Arduino could not handle it. You would not be parsing long strings or sending back a large amount text.
Another thing to consider is that for monitoring a simple sensor and transmitting the
data you probably do not need an Arduino. Radios that support protocols like Zigbee (which is used for sensor networks) usually have an embedded uC with digital I/O and
analog monitoring. The radio can be configured to periodically measure and transmit readings. Checkout the MaxStream XBee datasheet and the O'Reilly book "Making Things Talk". With a radios on the sensors all you need is a radio on your PC.
If you do decide you want an Arduino with a radio I make one with a socket for a MaxStream XBee --- Loading...
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