Whoa...the Volt-Amp rating really has nothing to do with the power supply - at least not in the way you're thinking. I've got 6 UPSs in the house, most hand-me-downs powering the kids' PCs and the home theater PC. I just recently bought a new APC 1500Va for $165 at Best Buy. I upgraded from a 650KVa. I have a 700W Active PFC PSU. The 750Va/310W would hold my system for about 6-7 minutes, while the 1500Va will hold it for 25-30ish. That's why I upgraded - because I needed more time. Previous to the 750Va I had a 500Va/240W (on my old C2Q build), but it would only hold it for about 4 minutes.
The Va rating is really more for measuring how long the UPS can hold a given system based on the wattage it draws. For example, my rig draws 230W at idle and about 350W at load. That's an overclocked 920, 12Gb memory, 160Gb SSD, a 1Tb WD Black and a 1.5Tb WD green, and a 5870 with an overclock. Oh, AND 3 24" 1080P monitors. 350W MAX with my 700W PSU. You may have a 600W PSU but it's HIGHLY unlikely, unless you're running dual GTX480s, that you're actually drawing 600W and it's probably less than half that. Without knowing your rig, you could compare yours to mine and kind of make an estimate as to your draw.
The APC BX1500G (the one I have) is 1500VA/865W. You probably don't need one that big, but it really depends more on how long you want your PC to stay up. I wanted close to 30 minutes and I got it on sale for $165 (it's now $199 - ouch). If you just need good surge protection with 5-8 minutes, you could go with a 600va model from whomever you choose. Want 10-12, go with 800va. Again, this depends on your PC so let us know your build (processor, memory, hard drive, etc...) and I can give you a rough estimate based on how long you want it to stay up in regards to how many VA you need.
The only thing you may want to watch in regards to wattage is to make sure it's got enough wattage to power what you need. A UPS with only 200W wouldn't do squat for my rig and would drain the battery in under 2 minutes, while a the 750VA/310W that I had worked just fine. As far as not finding ones with higher wattage, as the VA increases so does the wattage it can handle, so I suspect you're not looking in the right places.