Is there any way to run a guest x64 VM on Windows 7 x64? No luck with Virtual PC 2007, Virtual Server 2005R2. (both blocked during install). I know the Windows Virtual PC app that comes with Win 7 doesn't allow 64bit Guests...
Sun VirtualBox will allow you to create 64bit guests on a 64bit host.
There is a tool on that page to check if your processor is able to run 64-bit VMs in VMWare applications. Do you have VT turned on in the BIOS (assuming your processor supports it)?
For running x64 guests your options are either going to be Sun VirtualBox or VMware.
VirtualBox is free and generally pretty good, but as mentioned in another comment can be a bit on the flaky side.
VMware is IMHO still the best option for desktop virtualization. Workstation is top notch, but isn't free. The newly released Player 3.0 now lets you create and modify VMs, where you used to have to either use Workstation, Server, or an online tool to create and edit VMs.
If you can try both...they each have their own quirks. You get a lot more control in VirtualBox than in Player, but Player generally performs better and has more guest OS support.
There is a new version of Virtual PC designed for Windows 7 but alas, it doesn't support 64-bit guest OS's. Your best bets are VirtualBox or VMWare.
I have VirtualBox running a 64bit VM on a 32bit host (x64 CPU, obviously) but I can indeed confirm that it is capable.
QEmu comes pretty close to running anything on any host platform. It's not terribly easy to use, and using a different architecture than the host is pretty slow.