Some laptops released as recently as three years ago do not provide drivers for 64 bit windows. You may be able to solve this by enumerating every bit of hardware not detected by windows in the machine and looking on the original manufacturer's website, but the bundled utilities are unlikely to function in 64 bit windows. Depending on how useful those utilities are, this may or may not be a problem.
If the support website for your laptop has 64 bit versions of the drivers and utilities, there's probably little reason not to move to 64 bit, particularly if you have more than 3GB of memory installed (32 bit windows may have trouble addressing all of this). If you want to run some very old (16 bit) windows programs, you may also hit trouble, but there's XP mode for this, and it's only likely to be a problem in corporate environments with very old legacy software.
1The 64-bit OS will the 32-bit with compatibility libraries. I think what you may have heard before is that it's hard to find actual 64-bit versions of most software yet (at least on Windows). – Keith – 2011-06-02T04:43:38.840
1Oh, it does chew up some more memory when you mix 64-bit and 32-bit apps, since you will have two whole sets of libraries (DLLs) loaded. – Keith – 2011-06-02T04:45:45.047
1Remember that, SuperUser's tagging system notwithstanding, there isn't just one "64-bit" in the world. What may have been said about the Itanium doesn't necessarily apply to x86-64 (and vice versa, of course). – JdeBP – 2011-06-02T10:08:30.463
4The only thing you can't run is 16-bit apps, but it is exceedingly unlikely that you are still using any of them, and even then you could use an emulator like DosBox. – Callum Rogers – 2011-06-02T11:11:00.223
@JdeBP: I think the premise of a choice between 32-bit and 64-bit Windows excludes Itanium. – Ben Voigt – 2011-06-02T18:22:28.563
Not when it comes to things that "I've always heard", it doesn't. – JdeBP – 2011-06-02T23:00:30.057