If you've already got a Q6600 I wouldn't go "upgrading" to an E8400. Not that you wouldn't get better framerates in most games, because you would. At this point, though, barring the low-end, I've just stopped recommending upgrades for 775 systems. No point getting that extra 4GB of RAM (DDR2), new motherboard, new $200 processor, since it's no match for what you can get out of 1156. 775 is still good, but it's not something worth sinking money into, IMO. If someone's budget was really tight I might agree with an upgrade from a really crap processor to a cheaper C2D (E7400, for instance) but anything more than that is just not worth it.
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