Well, a bit more info is needed.
1. What format are you playing?
2. Do you have the deck posted in it's current build?
3. What sort of budget are you looking to work with, and how much do you already have invested.
Presuming you're looking to play Standard, I would say that a few packs of Innistrad would be a good start (a booster box is better if it's in your price range), as there are a good number of choice rares in the set. However, it cycles out of Standard in the fall, so you may want to stick with the current block and just get the stuff you need from the previous block as singles.
Anyway, I would generally suggest not buying more than two booster boxes of any particular set, and would likely lean closer to a single one, filling in gaps by buying singles. The reason is that a single box should get you a playset of every common in the set, give or take. Two boxes will net roughly a playset of every uncommon, and between the two you'll have at least one of most of the rares/mythics. Also, buying packs for specific cards is a waste of money...although it's fun to gamble on what you might get out of it, at the same time the odds of pulling any specific card in a set is fairly slim.
So yeah, if you can elaborate on the numbered points a bit, a bit more help may come along