With the exception of select select mathematical and design applications that benefit from 64-bit registers, such as AutoCAD, there is little performance gain from having a 64-bit foundation. In fact, most of the software you use on a daily basis does not have an optimized 64-bit version and may not for some time due to the limited benefits. Thus, at this time the primary benefit is its support for more than 4GB of RAM, and even at that the average user has not reached that limit.
On the other hand, 64-bit drivers, while becoming more prevalent, are still harder to come by. (Even Microsoft does not have 64-bit drivers for all of their current products!) In addition, some software, in particular antivirus utilities, must be designed for 64-bit computing, increasing the chance that you'll have to replace your current software package.
In short, 64-bit offers some performance gains and removes the immediate restriction of RAM capacity, but at this time it's not the best choice for the average consumer. I would suggest getting a 64-bit processor, but holding off on installing a 64-bit OS, at least for the time being.
P.S. Microsoft has a program where you can purchase a 32-bit version of Vista and then order a 64-bit DVD at the cost of S&H, thereby giving you the option without yet another large financial commitment.