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Hi Scott,
I've got a DGM NPB filter which works really well on nebulae - I think it cost about $150 USD for a 2", and tends to give a warmer tone to proceedings. Eta Carina will present the star as an intense vermilion colour, whereas OIII filters will give everything a greenish cast which doesn't really appeal to me. DGM NPB filters will also allow in some of the H-alpha wavelength (as well as OIII and H-beta), which may account for the warmer tone, and they've got a pretty good reputation as a narrowband filter. I've also used an Optolong CLS (city light suppression) filter, which while not as "narrowband" as an OIII filter provides a "similar" effect for less money. I was viewing Eta Carina at a dark(ish) site on the weekend and all 3 of these filters all made a huge difference. When doing your research you should try to track down the passband graph for each filter which shows the spread of light being let through - the narrower the better. (although the darker the image will be). I haven't tried the UHC filter that pgc hunter refers to, but I know there are a few reviews/comparisons floating around the net - a chap called David Knisley has done several reports which are very informative. I started with a CLS filter which cost less than $150 for a 2", which might be a decent starting point - generally the more "narrow" the bandpass, the more expensive they tend to get, from what I discovered. Others may have different ideas.
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