i used to have a pair of collareds, and i think the best thing you can do for them is to give them LOTS of space. They may only be uqite small lizards, but they have got to be one of the most active species I've seen!
Height is as important as floor space because they love to climb.
They're full of character and really cheeky once they have settled with you and are extremely friendly.
i used to let mine out in the room to chase crickets and sometimes they would run on their back legs, a really awesome sight
theres a bit of info here: Ians Vivarium International Reptile Community Forum | View topic - Collared Lizard Care Sheet