Actually an interesting question and one that I've addressed over the last 6 months.
I keep a dedicated XP/32-bit hard drive in my main W7 rig and boot off it ["bios dual-boot"] if I want to run any post W9x game that doesn't run or run as well, in W7 64-bit. Besides solving 64/32-bit limitation issues, I also disable crossfire on the XP setup to enhance compatibility - games of the early/mid 2000 era run just fine on a single 1gb card and sometimes balk at mult-gpu's.
After painstakingly checking all my XP/W7 era games there weren't too many that absolutely needed XP/32. The most significant one's for me:
Strike Fighters I series [especially the cool Korean Air War mod]
WW-2 Fighters [really a W9x game but likes the extra horsepower/glide wrapper options of a new system]
F1 Challenge [modded 1984-2005]
Steel Beasts Gold [2000]
Deus Ex 2
...and about 12-15 late 90's early 2000's point/click adventure games I like to keep around that won't run in 64-bit. I also moved games that dislike crossfire to the XP drive even if they run if W7 just to cut down on the fussiness.
None the less I was a bit surprised on how little I end up needing XP for gaming. I had expected XP to form the nucleus of a proto-retro build but now I'm not so sure.... Other than a handful of 32-bit games, and a UI which I prefer to 7, there isn't much that doesn't run or run better in 7 - or off one of the W9x retro rigs. For now the XP drive is really just a fail-safe backup if I ever loose W7 to hardware failure...
EDIT: I also keep 30-40 other games on XP that also reside on other W9x retro boxes because they run fine in XP/32-bit.