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Well if you don't mind buying used i'd recommend getting a powerware 9120 or a liebert gxt2 off of ebay. I grabbed a 9120 1000va for $150, even with the used batteries it powers 2 systems for at least 15 minutes (lansafe says it can go 27 total) with both under minor load (office, browsing, etc), replacement batteries are pretty cheap too. I haven't used lieberts software so i can't comment, I do like powerware's lansafe it works well and has client and controller versions so both systems will shutdown properly.
The two systems draw about 260w and 330va according to the 9120, the specs are #1 a64 3200+ @ 2.6, 1GB, 74GB Raptor, 2x seagate 300gb (7200.8 and 9), x800xl, audigy, tv tuner, dvdrw, 4 120mm, seasonic s12 500w. #2 sempron 3000+, 4 hdd 200-300gb of various ages, dvd, 9500 pro, enermax noisetaker 485 and a vx924 for the monitor.
If you want new and in the $100-150 range i'd recommend two seperate ups (the belkin 1500va that is on sale right now may work ok for two systems. I haven't used it or their software and I don't know if the software handles more than one system).
Well I found one of the ones you suggested on ebay for 22 bux (no buy out). I do not need more than 2 min of battery backup. We hardly ever lose power unless its just a 2 second power flicker knocking everything out. Both machines have a 45 day uptime as of right now and I restarted both of them for a recompiled kernel, I do not even remember when the power flickered. I don't even know if its worth it as this is not really mission critical. I am just toying with the idea.