if it's not much, I'd say go with 2.6, although I'm not sure what the $$ dif. is right now, and I'm too lazy to check pricewatch... but I'd say one of those two, then pick your mem. accordingly, but get one of the packaged sets form a good company, corsair, ocz, they're tested together and preform great
I gave in and checked 2.6 is 209 at googlegear, (best price w/3year warenty) 2.8 254, and 2.4 is 169, I'd say 2.6 is best deal
but if you wait until prices drop you could go higher or put the $$ to better ram or vid card