Is there a recommended upgrade path to migrate my AD data from 2003 (32 bit) to 2008 (64 bit)?
See the following:
In a nutshell, it's really no different than adding a new domain controller (DC, your new 2008 64-bit box), letting AD sync - then finally demoting, de-registering, and disconnecting the old server. This assumes that you have multiple servers available - or at least that you have a sufficient number of DCs available that you can remove one to re-install it with 2008, while not risking your existing infrastructure.
After you migrate - with no 2003 servers remaining, you'll want to consider upgrading your domain and forest to 2008 R2 functional levels, as they'll still be running in 2003 functional levels - even though they're on 2008 R2 servers.
Otherwise, if you're looking for an in-place upgrade of a (single) server, this is not supported across architectures (e.g. 32-bit to 64-bit). (Basically, this is the only concern on your 32/64-bit question - as it doesn't have any impact on any of the above.)