Aaahhh... the PSR-640 and PSR-740.
What I would say to "lokehinde" is congratulations on your purchase of the 640.  No, it is not a bad machine.  Have you wasted money buying the 640 because you know the 740 is better?  Probably not.  That's because the 640/740 are essentially the same, with the only major difference being vocal harmony, organ flutes, a few more voices, and 64-note polyphony.
So, if you don't do any vocals, you won't need vocal harmony, organ flutes - nice but not really necessary unless you're a fan of the real church organs, and 64-note poly is only fulfilled when you're recording complex sequences/arrangements using a computer's sequencer.
Marcelo -- still trying to decide on the 640/730/740?  I tried to help you with that, but it appears you're stuck with the 32/64-note poly thing... see above.  Again, the two digital displays and enhancements is enough to convince anyone that the 640/740 is far superior to that 630/730 series.
Hope this helps!
Bryan
ptshpboy@hotmail.com