The only accurate way to guage your UPS needs is to have a wattage meter like a
Kill-A-Watt meter. I am going to give some theorhetical examples to help you guage your needs.
Suppose your system uses around 100watts DC when idling and 200watts DC while under load (gaming, etc.) However DC does not equal AC. If you system uses 100watts DC while idle, your PSU consumes around 142.85 watts AC (assuming a AC to DC conversion efficiency of 70%). Also, I am assuming your Antec PSU has no Active PFC, but rather passive or no PFC at all your Power Factor (PF) would be 0.60 for example. If your PSU consumes 142.85watts AC, then it will consume 238.083 VA (what your UPS is rated by). For the 200watt DC load, it would be 285watts AC, therefore 475 VA.
Those numbers are only for the PC (theorhetical, your numbers will vary). CRT monitors typically consume 120-200watts AC, with a PF of 0.50. Again you wont know the numbers for your setup without a AC wattage meter like the one I linked above.
My advice? It sounds like the current UPS you have will be easily overloaded or wont last very long at all.