you really screwed it up man... i feel sorry for you. just make sure you won't commit the same mistake twice... hehehe... i also screwed up before... but not as bad as this one...
have you set your speakers settings to 4.1 in the creative audio console? if stil nothing happens, then you need to install the creative AudioHQ program, which will add to your control panel... and you set you speakers to 4.1 then test... if the rear speakers still wont work, then you must have...
ddr3 have faster speed, more expensive than ddr2... i think the price you pay for an expensive ddr3 is not yet worth it, because it evens out when it comes to the timings... but it will mature through time... better wait... til it matures.