Honestly, I would wait til perhaps they do a quarterly review(if they do such). I think the reason they gave you the $6 raise was due to you surpassing their expectations perhaps, but that's a very generous wage. Now, I'm over in Va so I don't know if $18 an hour is crap, but that is a gravy train over here. If anything, I'd ask for $2 tops. Did they say why they gave you the $6 raise? They might have given you the $6 because of all the duties you have listed. (And three months a is short time frame).
And I honestly wouldn't say greedy, but ill-timed. Wait as long as you can, until the economy starts to pick up, or maybe once they score a huge deal you helped them get, etc.
Source: I work at a corporate type office, and office politics can be a bitch.