I have AMD Athlon XP 1.2 Ghz, 1 GB DDR RAM. I don't know if I can install all drivers.
The 32bit architecture will do just fine for your PC setup.
Drivers are less of an issue, you would need some rather exotic hardware to not to be supported by either architecture.
As example: my workstation is an i7 2.3Ghz CPU with 8GiB RAM, but got by accident an 32bit installation of Ubuntu 11.10. And it runs fairly well - with the 8GiB used at max.
The Athlon XP CPU does not have support for 64-bit instruction thus you cannot use the Ubuntu 64-bit installation.
The Athlon XP is a K7 architecture CPU with support for 32-bit instructions only.