Personally, I measure flow by seeing what happens when I drop in some flake food. If much food falls to the bottom of the tank and can accumulate in spots without getting sucked into the filter intake, I think it's time to add more flow.
Personally, a tank this size I'd upgrade the filter before getting a powerhead. There's so little room in a 10gal tank I wouldn't want a powerhead cluttering it up (of course, I also just don't care for powerheads in general...) I'd go for an AquaClear 20 or 30.
cool, I have a tough finding purigen locally. does petsmart not carry it, mine ar always sold out. I have been using ROX 0.8 activated carbon from bulkreefsupply.com. Its good stuff, but would be interested in purigen if I could find it.