[CentOS] difference in x86 64 bit centos between 4.x and 5.x versions

i am new to the x86 64 bit centos versions.

ive always used the 32 bit version on industrial type HP hardware

for those of you that are running x86 64 bit centos, other than specific
hardware issues, are you finding that 5.x centos is better than 4.x centos
for x86 64 bit processing?

does it matter in the amd vrs intel hardware differences what you choose to
use for centos version?

other things to make note of?

if i need to be more specific in the general-ness of the approach, please
let me know.

thanks in advance for feedback.

 - rh