Might be a late reply, but for the record
I've been running WinXP as guest on a HP DV7 1150ed notebook. The notebook has no hardware virtualization support, so I can use only 1 processor of the 4. I'm on a pae.bfs kernel. Free command reports a total of 4098420 memory. I allocated 1024 mb to XP, because VB actually only sees 3GB memory. I have not activated PAE in VB. Just before I wrote this I increased the 1Gb to 2 Gb. Should not have done that. After starting Access I got a message that I need to reactivate because of a significant change. I needed the MS Office CD to do a repair (not a new activation). So, if you have to use MS-Office; install it after you set the definite machine settings.
After starting XP the Free command gives
Total 4098420, Used 3015124, Free 1083296, buffers 445472 and cached 773220
After loading a 400 MB Access database
Total 4098420, Used 3534512, Free 563908, buffers 630044 and cached 1112380
The performance of that particular database is very good.
On a different machine (i7 2600 with 8GB memory) I'm running Win7 64 bit using 3Gb memory (which is the maximum) and split the processors 50/50 between host and the guest. I get the full 8 GB on the host. Win7 is slower than XP. I can still do what I need to do, but especially file operations are slow. I e.g. avoid writing direct to a USB connected disk. I use a shared folder as temporary station. For this WIN7 configuration I would really like to see if a 64bit host makes a difference.