Rundll32 is just a host process that loads a DLL and calls a function in that DLL. The 'interesting' part is not the exe itself (which is part of Windows), but instead the dll it has loaded and what that dll does.
Sysinternals process explorer is your best bet on finding out what this particular instance has loaded; is it running some standard windows process/service, or is it hosting malware? The command line arguments it was started with will tell; first argument is the dll it loads, second is the entry point, followed by parameters passed to it.
As someone else mentioned: fiddler, wireshark, MS network monitor, etc may be useful in tracking down why it is using a lot of network bandwidth, and who it is communicating with. The "who" part can also be revealed with a simple "netstat -oa".
Finally, yes, on a 64-bit system you will have two copies of rundll32: a 64-bit version in system32, and a 32-bit version in syswow64.