As a very rough rule of thumb, the wattage rating of a UPS is approximately 0.6 * its VA rating so, as you have seen, a 700VA UPS is good for a power load of around (0.6 * 700) = 420W (your specs said 405W). Conversely, the minimum VA rating you need is approximately 1.6 * load wattage.

Your 700W PSU gives you some idea of how much power your PC will need 'fully loaded', but under normal running conditions it will perhaps use less than half this amount - it's hard to tell without knowing the full specs. And don't forget to add the wattage of your display if you want that to be powered by the UPS too.

Once you have worked out the maximum wattage of your load (the PC, possibly + display), you can choose the base VA rating of UPS you need - then you look at the specs and see which model suits your need. Here's where the fun starts, because you next have to look at how long you want the UPS to run when the power goes - and this can take two paths...

1. You can pick a UPS that is rated for pretty much the full VA you need so it will be running at 100% of capability and will thus last 'n' minutes.

2. You can pick a UPS that is rated at a much higher VA value than you really need so, for example, is running at 50% of capability and will thus last for longer than the UPS from option 1.

To complicate matters, you can sometimes buy extra battery packs for use with an 'option 1' UPS so its VA rating is not increased but it will run for longer....

...or you can overspec as per option 2 and know that you can add to the load on the UPS in the future, albeit at the expense of run time.

This is the point where you decide whether to read up on all this and become a UPS choosing guru, or use the manufacturer's UPS selection tools!