If you have a 700W PSU then it can potentially draw 700W from the UPS. Just like a PSU, even a UPS has an efficiency. UPS efficiency is very low compared to PSUs. They usually are rated at 65%.
However the real world value is much lower, around 50%. I also have a 1KVA APC BR1000IN which is 65% efficiency. Thus it should run 650W theoretically. The bloody thing never even ran 450W (according to their software's power reading). I had a 500W PSU at the time. And if the power went out it would immediately overload and cut off the power.
I then upgraded my computer and got a 750W PSU. It wouldn't even turn on on the so called 1KVA APC BR1000IN. Finally I bought an APC SUA1000UXI. Its 1KVA but 80% efficiency. It doesn't come with batteries, so you need to connect external LeadAcid batteries. It has a meter on the front which shows the load % and it seems to be true 800W capacity.
So for 700W PSU I think the minimum you need is the BR1500IN 1.5KVA. That one has built in batteries.
APC's home UPSs are apparently SHIT. They are made for the cheapskate Indian consumer who thinks he's very smart by getting a good deal just because the UPS casing looks better than their PC cabinet. Their inverters were such shit that it was removed from the market. Like I mentioned before, their 1KVA BR1000IN home use model didn't even provide 450W. However their server grade UPSs like the Smart-UPS series are excellent.
Another thing you must keep in mind is that 700W is exclusively for the PSU. You will probably be connecting monitor, speakers and other shit directly to the UPS as well. So you need to add wattage for those.
Expect to spend 10-15k for a UPS for your config. If your PSU is not fully loaded and isn't consuming 700W, then you can get away with a lower wattage 1KVA UPS for around 6-7K. But it will become a problem later when you start adding more components.
If a company like APC is capable of producing a piece of shit like the BR1000IN, then imagine the quality of other local brands like Microtek and Zebronics.