32 Bit OS on a 64 Bit chip?

Hi:

I have an AMD Athlon 64 4000+, 64 bit cpu.
I have a Drive Bay rack and switch between SuSE 10.0 64 Bit and Windows
XP/32 bit and Windows 2000/32 bit. I also have a Knoppix and an Ubuntu
Live CD, one of each.

Both Windows versions, Knoppix and Ubuntu FLY on this system! The speed is
amazing!

The SuSE 10.0 64 feels like when I was running Windows NT on an AMD k6 10
years ago...fast enough, but not what I was expecting.

What are the differences between a 64 Bit OS and a 32 Bit OS, as it
applies in this context (I know the difference between the architectures,
I want to know the advantages of a 64 BIT OS vs running a 32 Bit OS on the
same setup...)

Thanks!