I'll agree with everything above, but most important is to know yourself. How drained are you at the end of the workday? What feels right, and what feels like too much?
I took between 3 and 7 credits per term every fall, spring, and summer for two and a half years, in addition to a "day job". Only once was I really overextended, so it can be manageable. On the other hand, I stopped taking courses entirely this last spring to prepare for the MCAT (in addition to the occasional week off work to really hit the books hard). I found this very beneficial, and I recommend it. Often, the stress of juggling different activities isn't so much from not having enough time, but just from having too many things on your plate. If you can narrow yourself down to two major activities at a time, the stress can be much, much less.