$PCalc would be helpful.

The specific "ceiling" I'm trying to get to is 256^10, or 2^80. I'm dealing with encryption.

I wish I could say more, but the suggestion sorta speaks for itself.

Making a library to do that isn't really that trivial, quite hard actually. However, "something" like that could be done VERY easily with a dll. You could use the OpenSSL BigNum package. It doesn't support typing like "2 + 3" you'd do like BN_add(2,3) but it works on numbers of "infinite" size (infinite is defined by available memory and whether you want to wait 30 years to calculate adding 2 5trillion digit numbers :P)