Welcome to Flyertalk! ^
If you choose to upgrade (it is up to you to choose if the value for the money is worth it), you will as others have mentioned get a bigger seat with more leg room. Also you have more shoulder room, which is important to me. This means that unlike in economy, there is space between seats.

Flying in first class, you have priority boarding (for all the published benefits of first class, check out the appropriate section of the united web site at) http://www.united.com/web/en-US/cont...ght/first.aspx

Once on the plane, you should be offered a pre-departure drink of choice. Once in the air, there should be a round of drinks followed be a meal service, depending on the time of day. For information on what type of meal you might get, you could check out the meal discussion page in this forum: http://www.flyertalk.com/forum/unite...ess-class.html

On the A320, try to get seats in row 2. Row 1 has limited leg room and row 3 may have less recline, plus if the A320 gets switched with an A319, row 3 doesn't exist on the 319 and you might end up back in coach.

Some people would never consider paying for an upgrade, but for others, it is worth it. Really depends on your priorities!

Let us know what you decide, and even better, report back after flying to let us know what you thought of the flight and services! :-)