IFR AT72 AT 13000 FT EXPERIENCED TCASII RA WITH T38 IN A80 AIRSPACE.

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WHILE IN CRUISE AT 13000 FT; VMC; AUTOPLT ON; FO'S LEG; WE RECEIVED A TCASII TA WITH AN ACFT AT OUR 1 O'CLOCK POS; CLBING AT 1200 FT BELOW US. WE DID NOT SEE THE TFC; AND THE ACFT CONTINUED TO CLB. WHEN THE ACFT WAS 800 FT BELOW US; I TOLD THE FO TO 'GET READY.' ABOUT 3 SECONDS LATER; WE RECEIVED AN RA TO CLB. A CLB WAS IMMEDIATELY INITIATED; AND I THEN SPOTTED THE TFC COMING AT US AND CLBING. I CALLED 'MY CTLS' AND INITIATED A HARD L TURN TO AVOID THE TFC. TCASII THEN ADVISED US TO 'DSND;' AND I INITIATED A DSCNT. WE OBSERVED THE TFC; A T38; PASS OFF OUR R WING; AND I LOOKED AT THE TCASII AND HE WAS +01; OR 100 FT ABOVE US AND STILL CLBING. WE THEN ADVISED ATC WE HAD RESPONDED TO AN RA; AND RETURNING TO 13000 FT. ATC SAID; 'WOW; I DIDN'T THINK THAT WAS GOING TO BE A PROB. HE WAS AT 8000 FT AND NOT CLBING FAST AND THEN HE JUST SHOT UP.' FLT CONTINUED TO DEST AFTER TALKING WITH THE FLT ATTENDANTS TO MAKE SURE EVERYONE WAS FINE. WE WERE APPROX 20 MI W OF LGC ON THE ARR; ON FREQ 120.45(?); AND ATC NEVER TOLD US LOOK FOR TFC OR ANYTHING ABOUT TFC. I ADVISED ATC THAT IT WAS 'THE CLOSEST I HAVE BEEN TO ANOTHER ACFT IN ATL AIRSPACE.' WHEN THE TFC PASSED; HE WAS APPROX 1/8 MI AWAY; AND +100 FT ABOVE US.