Hi Yen,
I just ordered the Topeak MTX Trunk Bag from Amazon. You have to buy the rack, too.
My wife has a Topeak MTX EX Trunk Bag with the expandable panniers. She uses for runs to the store for small stuff, and for credit card overnights. It's a perfect size for a change of cycling clothes, a swim suit, and some going out to eat in public clothes. She also has a small bar bag which always stays on her bike.
I would not recommend beam racks that attach to the seat post. They have a carrying capacity of only 15 to 20 lbs.
I suggest a Topeak rack that is supported by braze ons at the chain stays as well as a seat post support ... like this
http://www.topeak.com/2007/products/racks/explorer.php
That rack will hold a set of full sized touring panniers if you ever decide to go that way.