First off, Parallels Desktop is currently on Version 7, so your version 5 copy might not run too well on the newer OS you are running.
I have a maching similar to yours, and my response is "what do you need?" I have 2 Windwos 7 Pro instances setup on my MacBook Pro (quad Core i7, 16GB RAM), and I installed both of them as 32 bit versions. One is for work remote support, and the VPN client they require doesn't support 64 bit Windows (I'm lucky to get Windows 7 support at all there), and the other instance is for running only a couple small apps, none of which need 64 bit, so I didn't bother giving either virtual machine more than 3GB, or more than 32 bit Win 7.