i have a very old APC Smart-UPS 600, the batteries were replaced after they were exhausted, but the electronics so far work flawlessly.
my pc has a corsair 750W psu and my cpu is core2 duo e8400, video card is hd4850 (just listed these, i think they consume the most signifficant part).
now im asking if will it be safe to plug the pc into that ups (by safe i mean it wont cause problems in the ups or electrical damage) - on the back of the ups it says 400W and 600VA - just for surge protection and just to be able to shut down the pc during power outages.
You'll be fine. Your system requires less than 400W and a UPS is protected against overloads. Don't connect a printer (laser printers should never be connected to a UPS) or your monitor to the UPS if you want longer runtime when there's a power outage.
You can also check from the UT/LCDdisplay of the UPS:UPSstatus,Input/output voltage&frequency,
And caculate if it al right for the UPS,also the UPS have a power factor,say 1000VA,if it's power factor is 0.7,you can only load 700Watt for power supply.
But the UPS will protect from overload by shut down.