well - I installed vegas pro 9b 64bit.
It installed and seems to be working fine so far.
I don't know how much faster the render is but it is using over 3GB of ram and all 4 AMD cpu cores are over 85 % utilized! I'll add an additional 7200RPM drive for media soon. Not bad for a $500 system !
I would opt for installing both, especially if you need to use some FX that is only 32-bit compatible. Unfortunately many filters are not yet 64-bit enabled. There is no problem installing both on the same PC
I would recommend installing both as well. For the plugin reason and if you don't need plugins, or if you've prerendered pieces that required plugins, you can later do a final render in x64 which seems more robust and faster than x86. I had to render a very long and complex veg that would just not complete in vegas 32, I prerendered a few parts that had plugin fx on them and then was able to render the whole piece in vegas x64. There's just no penalty to pay for having both on your system, at this point I wouldn't consider hard drive space an issue.