Since you are replacing the PSU, I can't see a power issue with a 4870. Make sure to go for 500w+ with one of those name brands just for a safety margin, and efficiency.
As for airflow, it will be essential to have good in-out flow on your case, as that card will dump all the heat into the air inside your box. 2x 120mm fans, one in front taking in, one in the back exhausting should be enough to keep it running cool. Fans are cheap, airflow is one of the most cost effective parts of a rig to optimize. Cable management is just about using plastic ties to keep the cables against the side of the case, to reduce airflow sapping turbulance caused by them hanging about. It is also cheap, and mearly a matter of putting some time and thought into it.