Well, thin is good, too little is bad. The idea of the thermal gel is to cover any micro cracks and whatnot. Thin layer evenly over the entire cpu surface ~ AS3?... i personally don't know if that is like AS5, but if it is there is a slight "curing" time, so the temps may go down as you use the comp a bit more (someone correct me if i'm wrong but 200 hours or something?).
If the ambient temp is in the low 80's, i don't think 40c is unreasonable, but would be more concerned with the load temps.
Is your full load temp still cresting 70? If it is, then yeah you may want to consider a better HSF, if it isn't, then i would chalk it up to the ambient temp (though a better HSF should always make a difference).
here is a good site to look into before getting a HSF..
http://www.frostytech.com/top5heatsinks.cfm
just scroll down a bit for the AMD ones.
I'm running a Sunbeam Core Contact myself and love it, it made a huge difference, though the thing is massive. You may consider measuring the area where the HSF will go to ensure it will fit.
as said before, good luck =D