The extra 15 watts on the 640A may come in handy with more demanding speakers. The 640A also has a higher S/N ratio for what thats worth and it uses a torroidal power transformer.

If your in the market for an AV receiver, consider the Onkyo TX SR 606 or 806.

You should also consider the Onkyo 9555 integrated amp. You may be able to find it on sale for about $500-$550.