Get as much as you can afford. Consider the following logic:

1) You will always wonder...
2) You can never have enough headroom...
3) It will curb the desire the upgrade, at least temporarily.
4) If you ever half to move to a larger room, you won't have to upgrade.

I don't know if you have the Bland over at the forum that shall remain nameless recommends spending $1-2 per cubic foot.