Most importantly you want a Line Interactive model, what that does is constantly monitor the line condition, boosting up the level during a brownout and switching to the battery if line voltage exceeds a certain amount, while not running on the battery 24/7 (which would be an Online UPS, intended for servers and hospitals), and having a very fast switchover (compared to a Standby model, which can have a switchover time in excess of your PSU holdup time). Also, make sure it is a Pure Sine Wave UPS, as square wave (also called Simulated Sine Wave) can cause damage to components, I blame mine for blowing my last PSU. Now they're common and not much more expensive than the square wave models.
Now comes the most important part: size. If you can afford a 1000w then get it, as it will allow for future expansion, compensate for battery life degeneration over time, as well as give a longer run time.
Also important is the brand you use. CyberPower, TrippLite, and APC are the big three. Of those, CyberPower models are the most affordable, especially their pure sine wave models (I run two, unfortunately not pure sine wave but guess what I'm going to get soon
, a 900w for my computer and 600w for my TV/DVR/Bose Wave Radio/Satellite Receiver). Before this I had an Ultra brand 600w and it...well...Let's just say I won't buy one again.
But UPSs have come a long way in the past couple of years, most even have an LCD display with diagnostic and other information such as in/out voltages, battery level and runtime, and load level, and all have a very high dollar connected equipment guarantee.
AMD FX-8350 w/ Corsair H105, ASUS Sabertooth 990FX Gen3/R2, 8GiB G.SKILL DDR3-2133, XFX HD 7970, 512GB Vertex 4, 256GB Vector, 240GB Agility 3, Creative X-Fi Titanium w/ Creative Gigaworks S750, SeaSonic X750, Corsair C70, HP ZR2440w, Win 7 Ultimate x64