Are you planning on the 55 being the turtle's permanent tank? For most of the species mentioned (RES, sliders, painteds), they'll need MUCH more space. The rule of thumb is to get a tank with 10 gallons for every inch of turtle. So a female RES, which can get 8-10", you should have a 75-125 gallon tank. For two of these turtles, you'd need twice as much.
Also important, one turtle doesn't need a turtle friend, many do better alone. There are some instances in which people are able to keep several turtles together but there are just as many cases of one turtle being bothered and tortured by another. On a turtle forum a while back, saw pictures of two turtles that had been purchased together, yet the larger had literally torn the smaller turtle apart.
I would also caution against just taking any wild turtle that you happen to find. Not only is this bad for the turtle, both in terms of health and the terrible stress, wild turtles are also likely to carry parasites and are typically more antisocial and fearful than turtles born in captivity. Try looking on some of the turtle forums, Craig's List (people often trying to sell tanks & the turtle comes free) or even contacting a local zoo. Also, avoid TurtleSales.com - more turtles arrive at the buyers door dead than alive.
Lastly, are you sure you want a turtle? Many people consider them more work than entertainment. . .
If you can't tell, I'm more of a turtle-person than a fish-person (also more of a turtle-person than I am a people-person). They can be awesome, but they're just not the right pet for everyone.