Q57 and i5/i7 are client platforms. They do have the basic support for virtualization (VT-x, aka CPU Virtualization), but is really intended for "light" use, such asone or two VMs with a single user type usage.
Depending on your needs, you might want to consider a Xeon based server platform. The recent chipsets have VT-D features (for I/O virtualization), and the processorsinclude the second generation of VT-x which includes additional support for memory virtualization and additional enhancments. The low end Xeons overlap the high end desktops in terms of cost.
The key question really is about your requirements and the intended purpose and throughput/utilization of the server.
As for Xen, it is highly optimized to work with the Intel VT features and I know plenty of people are spending a lot of time adding optimizations and features.. If you end up with a Xeon class machine, you might even adventure into the latest Xen (unstable beta for example) to get additional optimizations.
