Yes they are helpful. High performance memory usually comes stock with heat spreaders. One of the Abit motherboards comes with this neat air duct for your memory.
If you o/c(overclock) then i would say yes. for memory like you have it's not needed. it would still help though. heat reduces the lifespan of parts so any heat you can take away will help your hardware last longer.
Yes. Heatspreaders will always help. Especially if you overclock them or have voltage eating RAM (like the OCZ Gold VX 4000's or Mushkin Redline XP4000's). Usually more expensive and higher performing RAM will come equipped with them.