The main reason for 64-bit is memory. A 32-bit OS is limited to a bit over 3 GB of usable memory. 64-bit OS memory is limited by your motherboard. Some synths stream data from memory so many people want as much memory as possible. As more tracks are added to the project and more plugins to each track, the memory requirement goes up.
If your library of 32-bit plugins has worked for you so far and you do not create projects where this has been an issue for you so far, then there is no real reason for 64-bit other than future expansion.
It is largely a matter of what you use sonar for. If primarily for recording audio with simple mixing with a few effects, you could get by with a couple of GB of RAM and a 32-bit OS would do the job.
If you constantly produce loads of synth tracks with lots of plugins I would want 8-16GB and you need a 64-bit OS for that.
It is sort of like, if you have not run into a situation where you have needed more than 3GB of RAM or noticed it, then you probably don't.
As far as you plugin library, Sonar x64 will run most of the stock 32-bit plugins and many 3rd party 32-bit plugins without issues. Some 32-bit plugs do have problems in Sonar x64.
You can also run BOTH Sonar 32-bit and Sonar 64-bit on a 64-bit OS. If you have plugins that MUST have sonar -32 then you are still good to go. I run both.
I would probably advise you to get a 64-bit OS, install 4GB to start with and install both Sonars. If you need more mem, then simply install it, if not continue to use Sonar 32.
Michael