If you go buy a D-LINK DIR-615, make sure that it is hardware revision A1. Hardware revision B2 has less flash (only 4MB) and RAM (16MB) and a completely different processor: Ubicom IP5090U cpu. This CPU uses a new instruction set (ubicom32) that isn't supported by any open source operating system that I could find. This is also the previous generation of the ubicom processor. The IP51xx is the latest generation of processor, and evidentially its only used on the DIR-655.
A good place to look for information about what processors and radios are in the different access points on sale now, including version information, can be found over at small net builder with information about the D-LINK DIR-615 B2.