the 550 is fine for what you need - the 800 won't get you significantly louder, and if you get one of the 800s that can only drive 4 ohms per side, then the 550 will be able to drive just as many cabs.
I think folk have covered it, and you could always read the OHMs FAQ linked in the amps FAQ sticky but here goes. All bass cabs and amp outputs are wired in parallel. four 8 ohm cabs will give you a 2 ohm load, as will one 4 ohm and two 8 ohm cabs, as will two 4 ohm cabs. Three 8 ohm cabs will give you a 2.67 ohm load, as will one 4 ohm and one 8 ohm cab. A four ohm load can be achieved with one 4 ohm cab (duh!

) or two 8 ohm cabs. and of course an 8 ohm load will be from one 8 ohm cab.
You're not going to get a huge volume increase by going with a lower impedance (in ohms) if you do something like get the 4 ohm version of a cab rather than the identical cab in 8 ohms. What will give you more volume "bang for the buck" is using more cabs, or a cab (or cabs) with more drivers in them. Also the sensitivity and voicing of the cabs will affect your volume. (go read a bunch of threads in the Amps FAQ sticky for a lot of info)
on to the gear...If it were me I would stick to the 550 head rather than the 800, the extra cost just doesn't justify the small power increase, and you can use just as many cabs. I'll be honest, I'm not impressed with the nemesis cabs. If I were you I would check out a bunch of cabs (not just eden D series) - and find some that you like and that seem to have enough volume for you. I agree that a 210 or one 1x15 probably won't have enough volume for loud rock. When I used a WT400 (the same as the 550 but it can only go to 4 ohms minimum and has a slightly smaller wattage rating at 8 and 4 ohms) I used an eden 210XLT and an eden/swr 410t (both 8 ohms) This was a very loud band BTW. Later I used a bag end 115 and a bag end 112. I've played a friend's 550 through two 1x15 and two 1x12 bag ends (all 8 ohms so it was a 2 ohm load) it was loud as Poseidon.
I'm not saying you should use the cab setups I did, but i would recommend looking into getting two good 8 ohm cabs to start with, maybe two 2x10 cabs, maybe something else. You can always start with one cab and save up for another.