I bought the UPS just today, Microtek's 1050 VA. My config is the same as yours, except that I have DX58SO board, and 2 HDDs of 2TB and 320GB.
Normally, a new UPS has an efficiency of 0.80 and later it decreases to 0.65 to 0.60. So, 1 kVA UPS will initially be capable to run 800 W and later (nearly after 2 years) decrease to 600 W. You can still load it to 800 W, but there might be a loss in total backup-time etc.