Well... I have 2 computers, a 19" monitor, a kvm switch, a ethernet switch, a firewall, and a cable modem. All running off a APC Smart-UPS 1400. With everything turned on I run at 34% load.
I checked the site and it says that UPS is rated at 130watt output. The cable modem it will power no sweat... it may or may not power the Celeron computer.
My suggestion is if you are on a budget: Goto E-Bay
I've bought my UPS's used off E-Bay and got great deals. I got a Smart-UPS 700 for about $49 US (in great condition). I've seen smaller UPS's on sale for less there as well.
My final suggestion, stick with APC... or as a last resort Tripp-Lite.