Dual booting much easier if you have Powerquests Bootmagic.
You setup a small FAT32 partition for bootmagic, and 2 x NTFS partitions for OS's and whatever other partitions you require.
1) Install your non XP64 OS first.
2) Once installed format the small partition as FAT32, the other OS partition as NTFS
3) Install bootmagic to the FAT32 partition.
4) Configure bootmagic to not display the FAT32 partition and add the second (XP64) NTFS partition. Reboot.
5) On reboot select the 2nd partition (empty XP64 one), it will say that there is no OS installed and ask you to reboot.
6) Insert XP64 CDROM and reboot.
7) Choose 2nd NTFS partition again but this time when prompted select to boot from CDROM.
8) IMPORTANT - When installing make sure you choose the correct partition to install to. Also when given option select to "leave the current file system intact" . If you reformat Bootmagic can't find it anymore.
Dual boot using Windows itself -
Install non XP64 OS first.
Install XP64 OS next.
You may find that non XP64 OS looks like it not accessable.
In which case modify boot.ini file of the XP64 OS to include the non XP64 partition info.
Apps -
Dead easy, but ensure drive letters are the same.
If you wanted AH2 from both OS's install AH2 from one OS.
Reboot
Select other OS from boot menu.
Goto the AH2 directory and delete it.
Reinstall to same location under new OS.
That way both OS have neccessary registry info.
Works for any App you want to install.
Drivers - ATI's one is still beta for XP64
Mobo should come with all neccessary 64 bit drivers.