Yup usually around 50-75% in SMP supported OSes/applications. You won't get a performance boost in games tho, especially if you play CS/HL since that engine only supports one processor. Quake 3 engines support SMP as far as I know.. but they are becoming a rarity these days..
With d/l'ing and using the net and playin games I would suggest getting a single faster processor instead and invest that money on a gfx card.
But as said before, the parts are worth more than the $320.. so it's really up to you