I haven't bought a mc-303 yet, but i may buy one if the 909 and 808 are tweakable, as in tune, decay, attack, snappy, etc. Is it possible?

Nope. The mc-303 has only several different samples taken of both. If you want those parameters, you'll have to buy the novation drumstation.

Thanks for your help

yep you can and its going to be cheper, you can tweak the decay and cutoff resonace attack etc I have one there a great all round tool get it second hand and be carefull of the Knobs and key / what state there in.

A late addition to this thread:
With the MC-303 Controller software you can tune the drumsounds. It's not anything near a real 909 or 808, but it's a great addition, although the user interface is kinda messy and the program somewhat erratic... http://www.ping.be/insitu/insitusof.shtml

And a cheaper way of getting the 808/909 sounds than a Novation is ReBirth. http://www.propellerheads.se/

yeah, i have rebirth 2, and at the moment the best option is getting a drumstation, since i've been doing most of my music creation on the pc. i've just started moving on to hardware and i could sync the drumstation to my juno 60 and it's seq (jsq60)