I don't know where I found out I could, but I have Windows 7 64 bit installed on a Pentium 4 2.8 HT desktop. When I finish my upgrades (this week/next) every computer in the school will be 64 bit except one of the servers. This is also a Pentium 4 HT, but at 2.4. It's running Server 2008, but we have Software Assurance so can install 2008r2 on it. 2008r2 is only 64 bit.
So, obviously, can I do this?? There's very little information out there about it.