Firstly I can't understand why you are associating the clogging of the filters with going heavily planted? Wether internal or external going planted should make no difference to how long it takes for the filters clog up.
I have used internal and external with no real problems.
The reasons we normally go for cannister filters rather than internals are:
1 - Aesthetics
2 - More room for plants/Fish
3 - Greater media capacity
etc.
Not to do with clogging, its to do with the picture we are trying to create and also having good flow with larger media handling.
As for CO2 DIY will not work very well with a tank your size. It is best used for 20USG sizes. It is possible but you will need 2 or 3 bottles and change them intermittantly i.e. 1 - Monday, 2 - Wednesday, 3 - Sunday.
Water surface movement is a vital thing. The HOBsyou have will create too much movement, the Fluval will be fine. 2 fluval 3s or and external would be best for your size tank. What you really want is to hav a gentle ripple, no splashing and no waves. HOBs tend to make a lot of movement. Have the fluval so that its output is at least 2" below the water surface. Alternatively you can buy spraybar attachments for the fluval for a fiver from P@H or internet.
I would personally suggest getting an external cannister filter with a turnover of approx 2400lph. After media, hose lengths etc this flow will be reduced. If not then get a Fluval4+ Internal to go with the 3+ you currently have.
Andy
Thanks for your reply Andy, as you seem to be very knowledgeable. Please stick with me for a little follow up. My plan is to stick with the existing CO2 system and maybe add some dissolving CO2 tablets into the water from time to time. I know this is far from ideal, and will likely have minimal impact, but it's what I think I'd do for the near future (I am going to be buying a
Fluval Osaka 70 gallon 
tank after my floors are tiled, so I don't want to make too much other expense before this happens). In the future, depending on the life spans of my more herbivorous fish, I may be inclined to add a more adequate CO2 system.
Despite budget constraints, and back to the HOBs, the problem is that the media sleeves are getting filled up with plant matter so quickly that I see the water (and debris) returning from the middle, as opposed to just out of the sides (through the media). Hence, when I take the filter media out, it is completely full with plant matter (caked beyond belief). What I left out, however and quite stupidly, is that I have also fed my fish/snails zucchini a few times last week, which no doubt contributed to this vegetation buildup. Still, I have read elsewhere that people are reporting that they need to make replacements in their HOB sleeves one to two times per week, due to all of the plant matter. That can get expensive/inconvenient, and I'd rather shell out the money now, worrying about maintenance less, and not have to worry about the low quality and water agitation problems or my HOBs.
So, I have made a decision to invest in a cannister filter. What do you think about me buying the Fluval 305 external cannister and keeping the Fluval 3, while ditching those Wall Mart quality HOBs? Would the cannister flow have much less impact on CO2 as I think it would? Am I on the right track here, outside of the CO2 system issue that is?
If
you OR anyone can recommend a specific cannister model that you'd buy for my tank/plant/fish species circumstances, take a look at my set-up and get back to me (I realize there are some non-nos in my set-up):
55 Gallon Standard Tank (up and running for over 5 years)
Long Air Bubble, flex bar, across the bottom of the tank, only running half the time.
Turbo CO2 Bio System (should be used only up to 40 gallons....doh!)
Fluval 3 Internal filter
2 HOB filters (a Top Fin 60 and a Whisper 20-40; both have bacteria sponges, but both filters are cheap/cheesey)
Standard, 2 weak lights/fixtures (petsmart special with the tank and the Top Fin 60, as originally bought. I at least addded a nice bottom border and canopy to the tank, making it look less like a petsmart special)
Plants: Mulitple onion species (standard, dwarf, calistratum?, nana/natta?)
Various Anubias (cofeelia and nana)
4 moss balls
2 large unidenfied plants that are my most beautiful plants (look like huge anubias, but are different and no fish nibbles them). Seriously, I may bneed someone here to identifiy these big, braod and beautiful green leaves)
1 large java fern mat, slightly sunken into my medium size gravel)
2 swords (red and green melon)
So far (a couple weeks), only one red melon leaf has been half eaten, and one of the anubia nanas has been nibbled a bit)
I've actually had new leaves on the Coffeeelia and Green Melon spruot with out being eaten (yet)
My tip: Anubias coffeelia, standard onions, and moss is the way to go if you have silver dollars)
Snails: 3 - 5 assorted colored mysteries
Fish (yes, I am overstocked): 1 large bala
2 large silver dollars who are an ongoing challenge, but will be kept in this tank
13 Black Neons
A "few" ottos
A gold Nugget Pleco
1 Red Finned Shark that is small (will be transferred to new tank)
2 clown loaches - medium (will like to also transfer, but they are jumpers, like the bala)
2 beautiful german rams that would go to my new tank, with a bunch of buddies
And just for the hell of it:
The 70 gallon Osaka is to have huge amazon swords, moss balls, finer black substrate, and more carpeting type plants that my 55 gallon species would gobble up. I am looking to have it be a black water tank, with Blue Rams, a school of Cardinals, Panda Corys, The Red Finned Shark (if possible), Snails, and, eventually, possibly, (cringe) 3-5 Discus (if I consider myself able to do bi-weekly 50 percent water changes and only feed these fish once a day)
