If you're talking about installing a host, you should go 64 bit. It sounds like you're talking about a guest, though, and since you mention that rather small amount of memory, I think you'd be better off going with 32 bit for the guest.
The biggest advantage to 64 bit is native addressing of large amounts of memory, and you obviously won't hit the 4 gig limit. Plus I've heard of some cases where 64 bit can be potentially slower for certain applications.
I can think of no advantage for using 64 bit operating systems on a system, virtual or real, with less than 4 gig of memory.
Some reading material on the topic can be found at the Wikipedia and here.
While I would personally probably opt to just slap in the 32 bit guest, it also depends on what you're going to do...if you're really concerned, you should probably install both versions (as a minimal Debian shouldn't eat a lot of install space) and test it with the configuration you're intending to use the VM for and see what you can do with benchmarking. With your memory being low there's no clear way to go one way or the other...so if you're concerned you'd have to actually try testing both. Create three partitions, install the OS 32 and OS 64 in partitions one and two, and use a data partition (okay, four partitions, since you'd have to have swap space most likely set up too).