I am debating on whether to have my current radiator serviced or simply but a new piece. I haven't gotten a quote on a flush, straighten fins, and paint from a radiator shop but I am thinking this will be north of $100.

On Ebay, radiators are advertised new for $130:
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?viewItem&category=33602&item=7948830743&rd=1

The listing provides dimensions of 28x17. I am presuming the radiator to be the same size as stock since the ad does say it is for a 71 eldorado.

Is there anything particular I need to be aware of to be able to support a 500's cooling needs?

The radiator I took out of the '72 Eldo was approximately 31"x19" (outside dimentions) and four cores. Measure what you have before ordering and make sure you know whether they're measuring form the outside of the tanks or just the core.