A B757's nose gear did not extend so a go around was executed; the aircraft entered holding and used the emergency checklist for a successful extension. Then because of low fuel they declared an emergency before completing a safe landing.

Narrative

Just outside of the marker; on a visual approach. Put the gear handle down and only got two green lights; (both mains); no green light for the nose gear. Also; two amber lights; 'gear disagree' and 'doors'. I told ATC that we needed a vector off of the approach because of the gear problem. ATC climbed us to 2;000 FT and told us to fly the runway heading followed by a handoff to Approach Control. I told Approach Control that we wanted to get into a holding pattern close by to work on the gear problem. ATC climbed us to 4;000 and direct to a nearby VOR to hold west on 270. I tested the lights and tried cycling the landing gear a second time with the same result as the first. I referenced the QRH for the 'gear disagree' checklist which had us extend the landing gear using the alternate system. The landing gear came down with three green lights. I told ATC that we have our gear down with three green lights and that we were ready for the approach. The First Officer and I had a discussion in regards to declaring an emergency; and we decided that despite having the indication of the gear down and locked; if something were to go wrong during the touchdown and rollout; at least we would have the emergency equipment standing by. Not to mention; we were minimum fuel. So; I declared an emergency. The landing and rollout were uneventful. ATC told us to clear the runway whenever we could and to stop the airplane so that airport operations could check the nose gear for anything obvious that would preclude us from taxiing any further. Airport operations suggested that maybe we should have the nose gear down lock pin installed. I responded that I didn't feel that it was necessary. We taxied the airplane to the gate. Fuel on arrival was 3.9.