Really does depend which and what you are processing.

if your guitar has just a piezo mike you will benefit from a mike simulator of some type. the yamaha AG Stomp is good but only has two effects - delay and chorus.

If you want some other effects phase and flange etc there is a Yamaha acoustic stomp.

the eq on both these is pretty weedy - a graphic eq is always a help in detecting those nasty freqencies which pop up, and enhancing the actual frequency that you want.

some very very good acoustic guitarists - the late Eric Roche for one - seem to devoted a lot of ingenuity and the into the actual blend of the different components that make up your amplified acoustic
sound - check out the Fishman catalogue and Raven do a very popular source blend.

However I don't claim to be any kind of an expert. Don't buy anything on the recommendation of an idiot like me. The best thing is to go and watch some guys who have really got their guitar sound sorted out. I saw Alan Taylor a couple of weeks ago and was most impressed with the sound he has achieved. Go and ask. You will find most of these people eager to share their knowledge. Sometimes they give seminars at folk festivals. take a note book and sketch their ideas down.