allright, as you said 5th lvl spells I'm just going to Assume CL 10 for a base:
Good spells to Extend:
Lesser Vigor(Spell Compendium): Living Creature Touched gains fast healing 1 for 10 rounds+1round/lvl(max 15 rounds). For you extending would make it 3 minutes of fast healing for a 2nd lvl spell slot.
Vigor (normal) is a third level version that's fast heal 2 for 10+lvl(max 25) rounds. CL 10 would make that 20 roundsX2 extend = Fast heal 2 for 40 rounds (That's a potential 80 points of healing for a 4th lvl spell. Compare that to Cure Critical which deals 4d8+10(14-42) at same level.
Extended command is kinda humorous. Instead of doing what you say for 1 round, they do it for 2 rounds.
Any buff spell with a 10min/lvl duration changes from an hour and a half to 3 hours, making it a potentially good long-term buff. (See Invisibility for example)
Shield of faith is a good buff, but at 1 min/lvl it's too short to be a long-term buff and too long to bother extending for a single encounter. Same for most any min/lvl spell, unless you expect to be in multiple encounters over the course of say 15-20 minutes.
Anything in the PHB that starts with the words "Protection From" is a good take.
Divine Favour and Divine Power if you're expecting a long fight
Favour being 1 minute (2 extended), and Power being round/lvl (20 rounds/2 minutes extended at lvl 10).
Also I know you can't extend it, but Righteous Might (lvl 5) is the spell 2 cast if you wanna melee as a cleric. Either Followed or Preceded by Divine Power. Those two spells are really all you need to be a competent melee fighter. (and if you're gonna do that, don't waste any money on a belt of giants strength)
THEROC - As far as loosing the amulet goes, You could always swap it to another slot. Vest(body) fits the proper body-slot affinity, and would thus cost the same. That is, assuming our DM is allowing such things (If not I'll have to change my vest of con and robes of resistance)
I'm not really that up on cleric buffs, I usually do druids and sorcs..