You really should start fresh.
Be careful to write down everything you need. I am dual-booting XP and Windows 7 RC 64bit. I used Acronis Disk Director to add another partition. I then installed Windows 7 to the second partition (do not upgrade your XP).
I definitely would dual-boot, at least until your comfortable to switch over.
Another option is to pull out your XP drive and use a new one for Vista or Windows 7 x64 in case you need to switch drives for whatever reason, it happens a lot.