1) Pretty much all CPUs are now 64 bit
2) 32 bit Vista (and XP) will happy use all the cores on your CPU, but this doesn't provide as much benefit as you might expect.
Individual programs will only use a single core, so they won't run any faster than normal, but because different programs can use different cores, you can run more programs at once than you could on a single core.
Some programs, like Photoshop, are multi-threaded, so they can make use of multiple cores to run faster.
3) Don't bother with 64 bit Vista unless you need to use masses of RAM, because it needs specially written and signed drivers, some programs won't work on 64 bit versions of the OS either.
The only real advantage with a full 64 bit OS is the ability to address more than 4gb of RAM.
A small side note - whilst 32 bit Vista can theoretically address 4gb of RAM, that doesn't mean if you install 4gb in your new PC you will be able to see it all, for reasons that are many and varied you'll only be able to see & use about 3gb of the RAM, which makes me wonder why PC manufacturers install 4gb in new PCs but only install 32 bit Windows, thereby wasting 25% of the RAM.