Definitely upgrade to 4 GB. If you don't upgrade anything else, make sure you do it, especially if you plan to use it for the next 3-4 years. 4 Gb is starting to be standard.
I highly recommend the 4 GB of RAM because it makes the MBA 11" perform so much better. Sometimes I perform some virtualization with VMWare Fusion and it works much better with 4 GB of RAM. I think since you are going to keep the machine for sometime I would recommend you select the 4 GB option.
you may average 1-2 apps at a time but there may be times when you have 5-6 open. Also safari with flash install will at times gobble up more than 500k. $100 ... get the 4gb, you wont be able to upgrade in the future.