Will this UPS be sufficient?

[Thump553]

I want something that will shut down my computer safely in the event of a blackout. Hooked up to the UPS will be:

1) Compaq CQ5320f (Athlon II X2 240, 500 GB internal HD, GE Force 240, 400-500 PSU (I forget)
2) LCD monitor -24" (AOC 2436)
3) external USB hub
4) three external hard drives (optional?)

I'm thinking of a Cyber Power CP550HG, which is on sale here:
http://www.frys.com/product/6337051

Will this be sufficient? If not, what size UPS should I shoot for?