Assume you mean 32bit or 64bit for the VMs. Obviously 4GB+ RAM isn't a consideration for going 64GB in your case. I'd run 64bit unless an App specifically required 32bit. Of course, M$ R2 crap is all 64bit, so there is that.
If you are going to use this for labbing software to be installed in these vm's run what will be run when the installation is complete. An example is Blackberry express... known issues on 64 bit etc..
1 dedicated MySQL Server ubuntu,
1 Mailarchiva Server ubuntu
1 2008 MSsql Server winserver 2003
1 IIS.ASP.Net Server winServ 2003
1 SugarCRM Server ubuntu