Leprechanmonkey is right that there is no true cutoff point, but IMO the sweet spot for price/performance (after considering motherboard and RAM costs) lies with AMDs slower Athlon II X2 and X4 chips and the low end
Phenom II X2s and X3s. Those offer great performance for the price, but depending on what you want, they might not be fast enough for you so it might be wise to compare ...