I've got this 800VA Belkin model. Runs 2 Athlon systems with 17" monitors, no problem. It also has a warning system to let you know if you connect to many things to it.
http://www.staples.com/Catalog/Brows...e=1&Sku=486251
No matter which one you buy they won't run as long on battery power as they claim.
Here Staples has a 500VA for $50
http://www.staples.com/Catalog/Brows...Backup+Systems
Staples will ship for free if the order is over $50. Good deal since these are HEAVY.
3btech.net has some good prices on UPS's with free shipping but I don't know about the brand. A 650VA for $36 shipped.
http://3btech.net/powersupplies.html