Matt,

Once thing to watch for is accuracy ... how much "fudge-factor" is involved?  Most of the hand held units I have seen are only accurate to 10-15 feet.  That is pretty close, but if you are going around a 40 acre field plus or minus 10-15 ft can start to add up to a real number!

I did a quick search for GPS area measurement and got several hits. One of the first was for an Australian site about Garmin's GPS 76 ( http://www.ja-gps.com.au/gpsprod_gps76.html )  If you check the specifications it says this:

GPSMAP76 GPS accuracy:

* Position: < 15 meters, 95% typical*
* Velocity: 0.05 meter/sec steady state

DGPS (USCG and AMSA Australia) accuracy (when connected to GBR23 or similar:

* Position: 3-5 meters, 95% typical*
* Velocity: 0.05 meter/sec steady state

DGPS (WAAS, not available in Australia) accuracy:

* Position: < 3 meters, 95% typical*
* Velocity: 0.05 meter/sec steady state  Dynamics: 6 g's

So if I am reading all this correctly, at best it is accurate to about 10 feet 95% of the time.  Will that be close enough?  If not, you need to start talking about "precision" equipment with more accuracy.  You can get it as accurate as you desire ... if you are willing to spend enough money.