firstly a 700W PSU doesn't output 700W of power all the time, 700W is the maximum power your PSU can provide, whereas your UPS is usually only to provide enough power to keep the system powered up, so, unless you use the full output of your PSU you dont need a 700W UPS, however, matching the 2 wattages would be a good idea because you can then upgrade without having to upgrade your UPS, obviously, provided you UPS is sufficient to power your system
Edit: you may have being better to remain with your old 550W tagan because new tagan PSUs are no longer recommended by HWF