Personally I would have gone for 32 bit and would
always do a 'clean install' with an operating system: on either a (blank) formatted HDD or a newly formatted HDD-partition*
(*slightly more tricky to do depending on your skill level).
Dual booting is also possible so that you`re given an
F8 option during boot giving access to each O/S.
I would ALWAYS fully backup all personal data/files/pics. from your previous installation on optical discs or an external-HDD before attempting the new XP install.....Just in case.

The 64 bit version is ok but I believe is more suited to a faster processor/memory speed....64 bit vers. used to mean (some) programme compatability issues but not sure if that`s still the case.
Using the
Programme Compatabilty mode in XP should mean older progs. will still run under XP.
Glad you didn`t go for
Vista I have both here (XP-Prof. on Desk and Vista+SP1 on Laptop) and would choose to work with XP every time.....Vista just isn`t 'flexible' enough for me; and I have given it a fair-try.

You`ll surely find a visit here very-helpful with regard to XP installation and boot-options:
http://www.theeldergeek.com/