Is using a Virtual PC on a RAM DISK super fast? (putting the .vhd file in a RAM DISK)

It's debatable.

Performance will indeed increase considerably. But if you have enough RAM to create a RAM Disk capable of storing a whole image, know that both VirtualBox, VMWare and VirtualPC are capable of caching your images as data is accessed. So while the RAM Disk doesn't necessarily become redundant, its performance increase over a cacheable image will not be that great.

It's also a risk. A power outage or any other situation where your PC is accidentally turned off will destroy you image (or any changes you made to it since you last load it).

Finally, it will make starting up and closing down the VM session slower as the whole image needs to be read from disk into memory and later saved back to disk.

harddisk speed is the biggest bottleneck nowadays, there are other bottleneck also (e.g. drivers delay, OS architecture design for mechanical drive etc).