I just build a new pc ( i7 920 at 3.8Ghz with a Radeon 5870 ). That plus my lcd monitor should require around 500 watts under load. Correct? And less than 250watts while idle.
My 600va UPS won't keep with this new pc under load, but it will while idle (most of the time). Can I leave the pc plugged to this UPS all the time and hope for the best while under load, or it may damage the pc?
I didn't even try to use the UPS yet, afraid of any damage. But it's an offline ups, so I'm guessing it's the same as any power plug until there's any energy problems.
Is it safe? Or I'm better off this ups, and wait two months until I can buy a new one.
Well the most important thing to know in this in the Waveform Type which in your case is "Stepped approximation to a sine-wave", it is better to use Pure Sine Wave UPS on modern PSU's. They are a bit expensive but are good.
Your UPS will probably struggle to provide battery power to this equipment, however perhaps you can manage with what you got if you are only interested in safety.
Your UPS can probably manage just the PC (not the LCD) long enough to shut it down safely in case of a power loss.
If that covers your needs, disconnect the LCD monitor and and any unnecessary peripherals (routers, switches etc) from the UPS. Connect those to a surge-arrest power strip to protect them from spikes (APC make some good ones for very good prices). That should be enough protection for them in all but the most catastrophic scenarios (e.g. lightning strike).
Next make sure your UPS is connected to the PC with the data cable (USB or whatever), and set your power options, either in control panel - battery and power, or in the APC powerchute software (whichever of the two you use) to hibernate or shut down the system in case of power loss, i.e. when it is on battery power.
And of course the way to test this configuration, is to save your work, back up your data, and then pull the plug on the UPS and see what happens. It should only take your system 2-4 minutes to hibernate which your UPS should be ok for. Of course this is a bit of a risk to your equipment so please consider that before doing it.
Now, if you actually want to continue working with your system through a power cut, you will need to invest in a UPS with 1500Va or above for a PC with your equipment.