BE40 FLT CREW AT 3000 FEET REPORTS TCAS TA WITH VFR TRAFFIC 500 FEET BELOW AND ELECTS TO CLIMB CAUSING TCAS TA WITH IFR TRAFFIC AT 4000 FEET.

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I WAS PIC OF A BEECHJET. WE WERE TRAVELING UNDER THE ATL CLASS B AIRSPACE XING THE ATL 051 DEG RADIAL AT ABOUT 18 NM (OVER STONE MOUNTAIN) TOWARD PDK. ATC ALERTED US ABOUT AN OPPOSITE DIRECTION VFR TARGET 500 FT BELOW OUR FLT PATH. WE MONITORED THE TARGET ON THE TCAS AS WE APCHED; BUT NEVER OBTAINED A VISUAL IDENT. AT ABOUT 5 NM FROM THE VFR TARGET WE ADVISED ATC THAT WE WERE GOING TO MAKE A SLIGHT R TURN TO PROVIDE ADDITIONAL SPACE BTWN US AND THE OPPOSITE DIRECTION TFC WE HAD NOT YET SPOTTED VISUALLY. SOON AFTER; OUR TCAS ISSUED US A TA FOR THE OPPOSITE DIRECTION TFC. AT THAT TIME; THE COPLT (PF) INITIATED A SLIGHT CLB OF NOT MORE THAN 500 FT TO PROVIDE ADDITIONAL SEPARATION. AS WE BEGAN OUR CLB; I IDENTED THE VFR TARGET AS THEY APCHED OFF OUR L WING. WE IMMEDIATELY BEGAN OUR DSCNT BACK DOWN TO 3000 FT. AS WE BEGAN OUR DSCNT; OUR TCAS ISSUED US ANOTHER TA FOR AN IFR TARGET BEHIND AND ABOVE US AT 4000 FT THAT WE HAD NOT NOTICED. OUR CLB TO AVOID THE OPPOSITE DIRECTION TFC PUT US WITHIN 500 FT OF ANOTHER IFR ACFT. DURING THE START OF THE CLB THROUGH THE END OF THE DSCNT BACK DOWN TO 3000 FT; ATC WAS IN CONSTANT COM WITH OTHER ACFT AND WE WERE NOT ABLE TO ADVISE HER OF OUR DECISION TO CLB. IT WAS NOT UNTIL AFTER WE WERE NEARLY BACK DOWN TO 3000 FT THAT ATC CONTACTED US AND ADVISED US THAT WE HAD NOT TOLD HER OF OUR INTENT TO CLB; ONLY OF OUR DECISION TO TURN AND THAT WE HAD COME WITHIN 500 FT OF ANOTHER IFR ACFT THAT HAD US IN VISUAL CONTACT. I ADVISED HER OF OUR ATTEMPT TO CONTACT HER; BUT THE FREQ WAS CONGESTED AND WE FELT THE NEED TO PROVIDE ADDITIONAL SEPARATION FROM THE ACFT WE DID NOT HAVE IN VISUAL CONTACT. SHE THEN CLRED US FOR THE VISUAL APCH INTO PDK AND ADVISED US TO CONTACT PDK TWR. I FEEL THAT THE CONCERN OF THE ONCOMING VFR ACFT THAT WE DID NOT HAVE IN VISUAL CONTACT COMPELLED US TO TAKE ADDITIONAL MEASURES AND THUS ENCROACH ON THE OTHER IFR ACFT THAT WE NEGLECTED TO SEE. ALSO; THE RADIO FREQ AND THE AIRSPACE IS CONGESTED; ESPECIALLY UNDER THE ATL CLASS B; AND WE WERE NOT ABLE TO ADVISE ATC OF OUR INTENTIONS. I FEEL THAT THOSE ITEMS WERE THE CAUSE OF THE PROB. IN THE FUTURE; I FEEL LIKE A MORE IMMEDIATE RESPONSE TO ONCOMING VFR TFC WOULD BE MORE APPROPRIATE; SO AS NOT TO FEEL THE NEED TO TAKE A MORE DRASTIC APCH TO PROVIDE SEPARATION WHEN THE TFC IS MUCH CLOSER. FURTHERMORE; I FEEL THAT THE TCAS SYS WE HAVE IN OUR ACFT SHOULD BE TRUSTED. HOWEVER; I HAVE BEEN IN CLOSE PROX OF OTHER VFR ACFT THAT SUDDENLY TURNED OR CLBED/DSNDED INTO OUR FLT PATH OR DID NOT SHOW UP ON TCAS UNTIL THE LAST SECOND.