Hi Craig,
Very sound advice is being given focal length wise.
Here is another very good option for you. Sylvain has got his mits on some new TMB Planetary 6mm & 9mm eyepieces he is selling here for $50 each! I've had the 6mm for close to a year now (from somewhere else and a few more pennies), and just got the 9mm from Sylvain. These EPs have a 60 degree FOV which allows for a longer drift time through the eyepiece of an object if your scope doesn't track, and the eyerelief on both is amazing- you don't need to park your cornea on the eye lens:
http://www.iceinspace.com.au/forum/s...ewpost&t=66161
While it might seem that their focal lengths are fairly similar, with my big dob they give significantly different magnifications that warranted the purchase of the 9mm. The 6 gives me 333X which is way, way too much for most occasions. The 9 gives me 222X which is much more frequently useable. This totally concurs with what is being said about the atmosphere being our greatest limiting factor for magnification.
In your 12" f/5 scope, the 6mm gives 250X, the 9mm 166X.
These are by far the finest high power EPs I've used.
I wrote a short report on my new 9mm TMB in the above thread.