It shouldn't be any trouble and there shouldn't be any particular "loss". For 32 bit applications you run (automatically) in a very fast emulation mode and there is really no noticeable slowdown (maybe 1-3% on average - imperceptible). For 64 bit applications you can actually often get a performance *increase* (sometimes 10% or more, although often you get no benefit besides increased memory space). Most specific applications have no issues with a 64 bit environment, it's drivers for your hardware that you have to watch out for. With Vista out now a lot of hardware manufacturers seem to be ignoring XP 64 a bit more, which is unfortunate. But most major hardware should still be well supported on XP 64 and I'd personally recommend it over Vista (having had personal experience with both).
- Oshyan