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

the first generation of Intel Xeon's (based on P4 Prescott) that
supported x86_64 were only slightly faster in 64bit mode than in 32bit
mode, while the AMD Opterons were considerably faster. the newer Intel
Xeons (based on Core2Duo) are quite a bit better.

the latest Intel CPUs are generally faster at most/many things than
AMD's CPUs, however, AMDs CPUs may still do better at scientific type
programming that can't take advantage of the SSE3/etc 'dsp' style
functions. Servers are as much a function of memory and IO bandwidth
as anything, so there's many factors.

My opinion is to first determine a few issues.

How much RAM will it need? If over 3GB of physical RAM, consider 64
bit.
Are there third party applications you need to run on it? If they are
64 bit, that answers the question too. If 32 bit, are they supported
on 64 bit OS? Do they have different versions for 32 and 64 bit?

lots of good info to ponder back from those that responded.

what i a getting at is, why would one use x86 64 bit Centos version 4.7 when
one can get the same in x86 64bit Centos 5.2 or later version?

is there a reason to stay with the older version other than for specific
deprecated hardware issues or something else?

is 64 bit Centos 5 just plain better OR would some of you choose 4.7 over
5.x and why?

- rh

No, IMO. If you install a new system then install CentOS 5. Version 5 is
so old by now that all major applications should be supported there. The
usual YMMV apply.