When comparing four-core systems, Vista will give a huge speed boost to AMD's '4x4' platform (a high-end desktop two-socket platform.) This is because Vista properly supports NUMA, which is what AMD's multi-socket systems use.
Although it should only boost speed to what it SHOULD be in the first place. Again, this won't have a 4x4 system beat Core 2 Quad, it will just allow 4x4 to use its full speed.
*NUMA stands for "Non-Unified Memory Architecture", meaning each processor socket has its own set of memory. When a piece of information is in the memory for socket A, and a processor in socket B wants it, it has to pass through the socket A processor. When the OS knows about this, it can cache things better. When the OS doesn't know about it, the memory controllers have to do all the work, which is inefficient. Because all AMD processors have onboard memory controllers, if you have more than one socket, you have multiple memory controllers. On Intel systems, even multiple socket systems, there is one memory controller for all sockets. This means that on a massively-parallel system, AMD's system would probably be better, but even up to 4 sockets, a single memory controller will probably do better. (So AMD really should start sponsoring more supercomputing matrices.)