The APC Smart series all use sine-wave as I recall. I'm certain the particular model you have uses sine-wave. Just get new batteries, they seem to last two years then fail as a rule. I think APC builds some planned obsolescence into them, but that's another story.
Don't worry about your power supply being bigger than the UPS rating, you're not going to use anything close to the full capacity of your power supply. Its not like a 100 watt light bulb that uses whatever its rated at all the time.
Look at the load meter on the UPS to tell if its overloaded, you didn't cause the battery to fail by hooking up too much unless the load meter was full (and you probably still didn't). If the load meter is lighting up all 5 bars, the UPS is loaded to 81-100% of its capacity and you need a bigger one. Otherwise you're fine, depending on how much run time you want. Check when your computer is being stressed, as it'll use more juice during those moments.
Replacing batteries on that model is easy by the way.
