Approach Controller experienced a loss of separation while attending to an emergency aircraft; failing to secure visual separation in time to prevent an Operational Error.

Narrative

Both aircraft were on RADAR vectors for visual approach. Traffic conditions were heavy but generally routine until a C130 was handed off to my position in an extremely unusual position and altitude. This aircraft had declared an emergency (smoke and fumes) in the cockpit. I felt the need to get aircraft in vicinity down to 3;000 so that the C130 could be issued 4;000 ASAP. I did so with Aircraft X sooner than I would have under normal circumstances. The C130 contacted me at descending out of 14;000 FT approximately 15 miles north northeast of the emergency's airport of intended landing. I became distracted trying to find out what the C130 desired and other pertinent information concerning its situation. Subsequently; I noticed Aircraft X on a converging course with Aircraft Y. I issued a traffic advisory to Aircraft X; but it could not see Aircraft Y. I issued instructions to Aircraft X 'Turn right heading 260 and expedite turn'. It appeared separation was lost perhaps prior any instructions or advisories I issued. Perhaps a reminder to Air Traffic Controllers to turn aircraft first then try to get the aircraft to see each other; especially when the situation is not noticed until separation is lost or nearly lost.