For a lot of reasons: Windows 7 64 bits.
You might consider 32 bits on the computer with 4GB RAM, as you won't be able to practically assign more than 2 GB RAM to the VM (the 32bit version uses less RAM, also you only start getting the benefits of 64bit with 4GB or more), but frankly, just upgrade the RAM on that machine to 8 GB, and assign 4 to 6 GB to the VM, run 64bit on it, it's an inexpensive upgrade and it will make a world of a difference for running Parallels and the memory intensive apps you talk about.
Last edited: Oct 18, 2012