AVR stands for Automatic Voltage Regulation. It manages the electricity going to the computer and makes sure that it remains clean and at a constant voltage. Have you ever noticed the lights flicker in your house? That is what is known as a brownout. It is when there is an under-voltage of power. When a brownout occurs, it can cause damage to your computer hardware such as your hard drive, and can also cause your system to lockup. Utilizing a UPS with AVR technology prevents under-voltage and over-voltage from ever reaching your computer.

650 or 500VA?
VA stands for Volt Amps and 650VA(390 watts) and 500VA(300 watts) is the amount of power the UPS system can output for your devices. Deciding which you need depends on your system and how much power you need. If you get too little power and try to plug too many devices into the UPS, you can overload the UPS causing damage to your components. As a general rule, it is always good to decide how much power you need, and then go the next version available to allow for upgradeability.

APC and Belkin have great sizing tools that will help you determine which APC product would best protect your system.

APC - http://www.apc.com/sizing/
Belkin - http://catalog.belkin.com/computerups.process