Would not recommend TrippLite UPSes, from personal (and work) experience. Size wise, 1000VA-1500VA is probably the rating you want to look for for a desktop setup though
Other option too if you're trying to be really budget oriented, look for a used UPS on ebay (I would stick to APC here, but that's just me) and buy a replacement 3rd party battery for it. The batteries are pretty standard sealed lead-acid batteries with spade terminals that just clip on.