McGraw Hill
Mc Graw Hill: Circulation
An extensive page on the circulatory system in general. Scroll down to section 49.3 and you will find a simple comparison of the reptilian system to the other vertebrae systems.
Other
Hillfield Strathallan College: Evolution of Mammalian Circulation
An overview of the evolution of modern mamallian circulation. This is a good site since it puts the circulation found in reptiles in perspective. Compares it to systems in fish and mammals.