That tank would be a good size if you brought one of the turts back. The claw shaking can be a mating thing or a territorial thing. He could just be telling the other turt to get out of his space. At that size, you should be able to tell what sex they are. Here's a link that will show you the difference. Scroll down a bit to see the comparison pics.
http://www.austinsturtlepage.com/Info/info.htm
As far as filters, you could go for a canister filter like the fluval 404 or something similar. I have a fluval 4plus but with my 50 gal tank, it's not really keeping up and I'm looking to upgrade. The canister filter sits outside the tank, and the intake tube sits in the water. Lots of people here have them and will be able to tell you more about them. Maybe it'll help me make up my mind too
Read all the Stickys on this site. It'll give you the basic info on what you need for a happy turtle. Check out the sections on lighting too. Do they have a basking spot? Fill the tank up as much as you can without them being able to climb out. The more water, the better, especially in a smaller tank.
The things that come to those who wait may be the things that were left by those who got there first - Steven Tyler