"Up to 4 times faster".. so says Apple. You can choose to believe that if you want.
In REAL WORLD situations, I have no doubt that it is faster than it's G4 predecessor, but I think 4x is a serious stretch. And then to be hampered by a lack of real video card.. well, anything that uses GPU will be slower than it should be.
However, the POINT of a Mac Mini is to offer a cheap solution to get Windows users into a Mac.
The Intel is not a celeron, it's a Core Duo. More info here:
http://www.intel.com/products/processor/coreduo/demo.htm