The UBB can take hundreds of megs, if you allow it. Assume that you need one meg for 200 threads.. this is generally true for UBB5. UBB6 is different, but we'll tackle that later.
You'll probably want about 50 megs of space to start with. If you run low, you can purchase more from your host, or prune threads from your UBB. (Or clear the cache in UBB6...)
Customized graphics and styles are built in features of the UBB board, and are generally very simple to set up.
Installation of hacks can range from simple to difficult, depending on your ability to follow directions and the ability of the hack author to properly document the hack. Knowing Perl (or another C-like programming language) helps.

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Liam: "...The only part I can't get to work is the actual equation: Poisson(X) = (A^X x e^-A)/X!"
Tim: "You've a mathematical function named w/ the French word for fish?"
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as overheard on Flare.