64bit has no support for anything pre-32bit.
For example. Quake 2 is 16bit. Millions worldwide still play it, but not on 64bit, it just doesnt work. Many other games and applications use the same technology. They will not work either.
I beleive (but as always i stand to be corrected), Windows 98 to XP emulated the 16bit dos subsystem to give compability to applications that required this. Vista 64 does not do this so anything that needs this simply will not run.
This was done using a windows process called wowexec.exe, its a process driver for emulating a 16bit application on a 32bit OS.
Drivers and patches have been slow out to support 64bit. My mate cant get his XP machines to print through his printer because the drivers dont support printing from a 32bit OS through a 64bit subsystem.
For this reason, stay with vista 32. Its essentially the same OS but has much better compatability.
paul