I bought a Belkin F6C-650 UPS for $110 and have been pleased with its performance. It's USB, 650VA (400W) for 45 min, with AVR. I have my PC (Athlon + 2 HDD + 2 Optical drives + lots of USB powered peripherals), 19" CRT, USB hub, cable modem, a radio, and VCR which only load it to 55% of capacity. Works with Win 98, 2k and XP (natively in XP) but not Linux. The lack of Linux support (for unattended saving/power down) is my ONLY complaint.
APC's are what I'd recommend but the Belkin was a more affordable option.