You would need a new motherboard, ram, etc. as well. Is $225 your whole budget or only for the CPU?
Or you could get an AMD PhenomIIx4 955, a crossfire capable AM3 790Xmotherboard , 2x2 GB of DDR3 1600 RAM, an Antec case and a 400w Antec power supply at the same price ($415). It's not as fast as the i5 750, but if you're still using a P4 you're probably not a power user.