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Strictly speaking, almost all 32-bit processors support at least 64-bit floats - in fact, the P6 and above support 80bit floats internally to reduce rounding erros. What you do see with a "64-bit" system is a 64-bit *integer* (long long) type in hardware. I wouldn't be surprised if OpenSSL made use of that fact.
Apart from tricks like PAE and whatever is in Panther that does the same thing, your memory addressing comment is quite correct. An Opteron running Linux x86-64 is a 64-bit system. A Xeon with 8 GB of RAM - and using it - is not.