Your title says Q8400.
I have a Q9400 and it will OC to 3.8GHz easily, as will the i5-750. At those speeds the i5 is still around 20% faster than the Q9400.
At this point the intel quad cores are relatively expensive. If they fit in your budget then great, but from Tom's recent $750 gaming build it seems that the less expensive AMD chip offers good enough performance for gaming and saves you money that would be better put into your graphics card(s). And don't get me wrong, I'm NOT an AMD fan but for some people they still make sense.
if you are conscious about power consumption, go with the i5-750 (95w TDP) if not, get the Phenom II (TDP 125w). the perfomance difference aint that much therefore you probably wont even notice it during gameplay.