Is there any reason to run 64 bit Vista on a system that maxes out at 4 GB RAM?

The biggest advantage of 64 bit over 32 bit is the ability to use large amounts of RAM. The 32 bit OS is limited to 4 GB but 64 bit Vista can support from 8 to 128 GB depending on the edition (Home Premium supports 16 GB and Ultimate supports 128). However, you can only use as much RAM as your computer's motherboard will support, so if the hardware limits you to 4 GB, the 64 bit OS won't help you memory-wise. In fact, it may hurt you a little, since 64 bit Vista can use about 12 - 15% more RAM for the OS and services.

I have found 64 bit Windows to be generally more stable than 32 bit on the same machine, but that advantage can be outweighed by the fact that it's harder to find 64 bit hardware drivers, so some of your peripherals may not work. If a driver isn't signed, you can't load it on the 64 bit OS. This is probably one reason for the extra stability, but again makes it more likely you won't be able to use some of your devices. Another consideration is that 64 bit Vista won't run 16 bit applications. So if you still have a few old 16 bit apps you want to run, the only way you'll be able to do that is to install a 32 bit OS in a virtual machine and run them there.

You also have a performance advantage when you run 64 bit programs on the 64 bit OS. Unfortunately, right now there aren't many 64 bit applications; however, Microsoft Office 2010 will be available in a 64 bit version. Overall, though, with a machine with limited RAM, you're probably better off sticking with the 32 bit OS.