if you're not under any time pressure, why not setup the new serve with 64 bit and then migrate ISPConfig to it... that way you get more experience doing that.
The problem with 32bit is that you can only address 4 GB ram in there. Although some distros provide PAE which ehances the 32bit addressing to 34bit which means that you can use more than 4 GB ram however each process will still be bound to 4gb at max.
As you have 12 GB on the new machine, it would be a shame not to put it at good use.
"Common sense is not as common as commonly believed" by sjau