Are there any advantages for 64-bit boinc (NOT seti application)?
Advantage is that you can build totally 64-bit environment.
Dis-advantage is that you can't participate in many projects ... not if you don't want to play with
anonymous platform. For some projects (CPDN for one) even that doesn't help.
Is there one for Linux?
Officially no. There are some available on the net (Augustine publishes it quite heavily) and you can always build your own (the process is quite straight).