a 32 bit OS will only bottleneck your PC's performance if:
1) you're using more than 3 GB of memory*
2) you're using a storage array of more than 2TB
3) you're trying to use 64-bit capable intensive computing apps for things like cryptography
64 bit will alleviate those problems, allowing for support of more memory, larger storage arrays. The downside to a 64-bit OS is that it drops support for 16-bit legacy applications, and the emulation at this point is dodgy. A lot of older, especially obscure or custom software will not work with 64-bit vista.
*Of course, considering that with Vista, anything -less- than 3 GB of memory will slow you down, I guess it's a reasonable deduction that you need the 64-bit version so you can add more memory.
Oh yah, and I know a guy who -does- have a 6 GHz dual-core processor... and it's awesome... it's just that he spent more money on a cooling system for it than I make in a quarter.