Almost all 32 bit apps will still run fine with very few exceptions, however ancient 16 bit applications will not run anymore (but there are hardly any 16 bit apps left in the wild anyway).
I had a boss that tried switching a few of our workstations over to 64 xp. As a result, some of our applications, like 3dsmax (64), were terribly unstable. This was back in 2007; things might be different now.