are you going to be installing to hard drive? If so then no you wouldn't use a nanbsd/embedded image
have you looked at
http://www.pfsense.org/index.php?option=com_content&task=view&id=43&Itemid=44
This goes over all the different versions
example
Embedded (NanoBSD)
The embedded version is specifically tailored for use with any hardware using Compact Flash rather than a hard drive.
Architectures
Starting with pfSense 2.0, there are versions for both i386 (32-bit) and amd64 (64-bit) architectures. This architecture is noted in each of the filenames for download. If you are unsure which version to use, then use i386. It is the most mature and well-tested architecture, and it will work on both 32-bit and 64-bit capable systems. The amd64 architecture (which does work even on Intel 64-bit CPUs) can address more memory and may have other performance advantages, but requires a compatible CPU.
Now with that 8GB ram you have - if you want to have full use of that 64bit should be used. But 8GB seems a bit high for a pfsense box.. Are you planning on running stuff like squid and snort, ntop, etc.?