BE33 pilot experienced portable GPS failure while attempting to over fly ATL Class B airspace. Reporter drifts below 12500 FT during troubleshooting and is asked to contact ATL TRACON upon arrival.

Narrative

Shortly after takeoff; realized a failure of portable GPS. Reset GPS several times to no avail. Retrieved back-up GPS. Had some difficulty because GPS had somehow been switched to 'automobile' mode. Simultaneously encountered clouds and had to climb to maintain VFR. Approaching Atlanta outer ring; thought I was close to 12;500 FT. About the time I hit the outer ring; noticed that my transponder was not working. Found that circuit breaker had tripped. Reset breaker and transponder began working again. Glanced at GPS. It indicated altitude of 12;600 FT. However; altimeter indicated less than 12;500 FT. Climbed until altimeter showed above 12;500 FT. Never dropped below 12;500 feet until exiting Atlanta Class B airspace. When I attempted to contact Daytona Beach Approach; discovered I had a radio problem. Could receive well; but transmission on both radios was garbled. Approach could barely hear transmissions. Diverted. Upon landing; was given telephone number in Atlanta to call regarding 'flight deviation.' When I called number; I discovered I had actually inadvertently entered Atlanta Class B.