Hi,
acheryl, and welcome to the forums.
The biggest thing to worry about is drivers for hardware in a 64bit OS- in terms of software, the 64bit OS will allow the use of 32bit and 64bit software to run.
Now, that doesn't mean all 32 bit software will automatically run. Some older software, especially those written before Vista was a gleam in Bill Gates' eyes, were written to use full access to manipulate resources on the system - IOW, they gained access to devices that Vista now restricts access to - this is not a problem of 32bit versus 64bit - it is a problem of older, poorly written code and Vista in general.
There may be a few software that will run in XP that simply will not in Vista 64, but those are few and far between, Also, another major issue is that in Vista you need to have signed device drivers, so most likely 3rd party drivers will not work.
That is also something to keep in mind whenever you buy a new device - research it first on the Internet - will it work with Vista 64bit?