>>> would a recirculating hood such as a Broan cbd330 make much of a difference in grease and odors vs no hood at all?<<<
>>>I also know what CO is and why I should care about it.<<<
>>>Our house is old and far from airtight.Is my hunch that this recirculating hood won't do much good right?
Would it mitigate eau de cabbage stew any better than cracking a few
windows?<<<

My initial reactions to using that Broan without venting to the outside are these: that your Broan may help a bit with some aerosolized cooking grease (although the hood design means that a lot will not be captured); that the Broan may serve as a surface for condensing vapor but will be otherwise ineffective in dealing with vaporous combustion byproducts and steam, much less the odor/stench of boiled cabbage; and that it will do nothing about CO which you should not need to worry about, anyway.

For removing odors when boiling cabbage etc., you might have better luck with the windows, particularly if you can toss-in a box fan or window fan for those occasions.

CO emissions from modern gas ranges should be barely detectable with sophisticated equipment -- far below hazard levels --- unless somebody has really screwed with your burner adjustments

The combustion byproducts from the burners are rather more smoglike with the principal components being CO2 and NO2. The Dept. of Energy in conjunction with the EPA published a study on air quality in kitchens with "unvented" gas ranges and the effectiveness of realtively affordable venting a couple of years ago. The result was some overblown media attention. It also got some interesting commentary here which you can find at this link if you are interested:

http://ths.gardenweb.com/discussions/2304651/ny-times-the-kitchen-as-a-pollution-hazard

The idea of getting a convertible hood and later installing outside venting is not a bad one. I'm guessing that you are not currently venting because your stove currently sits on an interior wall and you all have been stumped on how to get outside venting without a full kitchen remodel to relocate the stove. Is that correct? That's a problem I had with my old-house kitchen and I discovered numbers of relatively easy ways to go up from a vent and run ducting over to the nearest exterior wall through a cabinet. This was easy for me because the interior wall behind my stove butted into an exterior wall and because I didn't mind the somewhat industrial look of the ducting. Might or might not be that easy for your old house-kitchen and the look might or might not work for you.

Having said that, the 30" Broan cbd330 may not be the best choice. It seems to be a low-rise hood which does not project very far out from the wall, and it has a flat base rather than a canopy. As kas has often pointed out, an actual canopy gives better coverage and capture and can be a significant part of the effectiveness of a range hood. Even better when the hood extends a bit wider than the stove. (Better still when the hood also has enough depth to extend as far out as the front of the stove, too.) Basically, the flat base on the Broan means that the unit depends entirely on fan-power to collect vapors, grease, and etc., The Broan has relatively low fan capacity. (250 CFM?) You might want to consider taking that unit back and swapping it for something with a canopy and, if it will fit, something 36" wide.

Again, that may or may not work in your kitchen. My old-house kitchen was such that I also had to resort to a low-rise hood with a flat bottom rather than a canopy. We work with the kitchens we have.