No, you're not stupid.
Basicly yes, 64bit allows larger adress spaces and thus more ram... but also 64 bit integer calculations are speeded up and many Databases, scientific and some games will benefit from that.
For the Athlon the advantages is that it will feature a revised and improved CPU architecture... getting rid of the old bad bit s of x86 and replacing them with modern designs... thus a faster CPU will result.
Think 8bit to 16 bit, and then 16 to 32 bit transitions...
