10k Mana's good, but slight overkill for heroics.
A good rule of thumb - if you're set for Kara (6.5k health, 8k mana, 1000+ healing, 100mp/5 while casting) you're golden for heroics. With a decent group, you can get away with a little less.
To boost the bonus healing ... 3/3 Primal Mooncloth is VERY sweet. If not a tailor, snag both pieces of Whitemend. You won't be able to use the set bonus, but the items themselves are very nice. Now, if I could only get my grubby mitts on them ... :P Would make my almost 1500 bonus healing even better. :)
[Edit: Grammar > me sometimes. ]
Edited, Mar 10th 2008 12:05pm by Shamandin