more or less than 500 teraflops?

well, it would depend on the computer. current quantum computers only run at a few flops. but as the technology advances we will be able to build them with faster operating speeds.

I believed that the idea of qunatum computing (at a more mature stage, of course), is that there wasn't necessarily a limit at all.

 

Correct me if I'm mistaken, but I remember an MIT professor explaining that a Quantum Computer of several thousand entangled particles would process more information than a classical computer the size of the known Universe.

You might have a look at Seth Lloyd's "ultimate laptop"

 

http://arstechnica.com/wankerdesk/01q2/limits/limits-1.html

 

more or less than 500 teraflops?

 

Certainly more. Sun is already building a 500 TeraFLOPS supercomputer called Ranger. Given past trends we can expect that kind of power on our desktop in 5-10 years.