Will 64-Bit Windows OS run in Parallels

Will you be using any 64bit applications? Otherwise not much benefit will be seen.
Personally I would use the 32bit if you are going to be using within a virtual environment.

Parallels supports just about every OS that can run on an x86 platform:

http://www.parallels.com/eu/products/desktop/sr/

+*Supports 32- and 64-bit virtual machine operating systems such as Windows, Linux and Solaris — not included.*+

+*Guest OS*+
+*(32-bit) Mac:*+

+** Mac OS X Server 10.5.x Leopard*+
+** Mac OS X Server 10.6.x Snow Leopard (experimental support)*+

Windows:

+** 3.1*+
+** 3.11*+
+** 95*+
+** 98*+
+** NT Server*+

+* *+

+** Server 2000*+
+** Server 2003*+
+** Server 2008*+
+** XP Professional and Home Edition*+
+** Vista Business, Ultimate, Enterprise,*+
+*Home and Home Premium*+
+** 7 (experimental)*+

+* Linux:*+

+** CentOS*+
+** SUSE*+
+** OpenSUSE*+
+** Red Hat*+
+** Red Hat Enterprise*+
+** Debian*+
+** Fedora Core*+
+** Mandriva*+
+** Ubuntu*+
+** Xandros*+

+* Other:*+

+** FreeBSD 7.0 & 6.2*+
+** OS/2 Warp 4.5*+
+** eComStation 1.2*+
+** Solaris 10 & 9*+
+** MS-DOS 6.22*+
+** OpenBSD 3.8*+

+*Guest OS*+
+*(64-bit) Mac:*+

+** Mac OS X Server 10.5.x Leopard*+
+** Mac OS X Server 10.6.x Snow Leopard (experimental support)*+

Windows:

+** Server 2003*+
+** Server 2008*+
+** XP Professional*+
+*Vistas Business, Enterprise and Ultimate*+

+* *+
+* Linux:*+

+** Red Hat Enterprise*+
+** CentOS*+
+** Fedora*+
+** SUSE Enterprise Server*+
+** OpenSUSE*+
+** Ubuntu*+
+** Mandriva*+

+* Other:*+

+** Solaris*+
+** FreeBSD 3.8*+

i installed RCwindows7 64bit in VMfusion on a 13"UMBP with 4 gigs of ram. it works, but unless you have 8gigs of ram its worth is questionable. The virtual machine (win7) will end up reserving whatever amount of ram you allocate to it and blocks the host OS (OS X) from being able to use it. not sure how parallels handles the ram, but i may be similar. 64bit in boot camp was able to use all the ram and worked well. i ended up reverting to a 32bit version of RCwin7 because there were some issues i had running older 32bit software in the 64bit win7. also i felt i wouldn't be hampered by that ram limitation of the 32bit os.