I don't really suggest an undergravel filter at all, as I've heard that the mulm and sludge will build up underneath your gravel, causing bad things to happen.
As far as filtration, are you looking for a canister filter, a HOB filter, or did you have something else in mind?
As for as the canisters go, I use an Eheim 2213 and it's excellent. I used to have cloudy water, but it's no more with my Eheim. It runs about $69.99 from Big Als which might be a bit expensive for you, although I'm not too sure what you're shooting for. I've also heard pretty good things about Rena Filstars which the XP1 runs about $59.99 from Big Als. Canister filters are ideal for a planted aquarium because they don't make quite as much surface disturbance as a HOB, and thus you won't lose any CO2 you may inject with plants.
If you're looking for more of a HOB filter, Aquaclears and Penguins or Emperors are all good filters. I personally have used a Penguin 150 on my 29 gallon, but it seemed to be far undersized for the job. The other issue with the Penguins is the pre-sold cartridges have activated carbon in them. Activated carbon is kind of a mixed opinnion thing around here, but it'll take out some nutrients for plants that you might want to have in there.
Your problem may also not be just your filter. Is the water green cloudy or white cloudy? Take a little bit of it and put it in a white cup if you can't tell. You might not be doing anything about the source of the problem or a potential imbalance if you just buy a new filter.
