Just spotted your thread and there's lot's of good advice. However, I'd also add the following;
Working out maximum load is OK to an extent. It's really about knowing the load on all seperate voltage rails. Quality suppies have several rails and therefore are far better at load balancing.
Having already had issues, and your intention to build a new machine in future, why not replace the supply for a quality unit at around £80 - £100 which can be used for the new build.
A UPS is a great idea and I use several APC units. You can get some very good domestic, single pc units.
At an absolute minimum you should be plugging into a Belkin surge protection strip.