If you have an i7 920, there isn't really any need to change CPU, motherboard and memory when all that money would be better spent on more GPU. That's anywhere from $400-600 worth of hardware for a suitable upgrade. Better off buying 2 instead of 1 of a high end VGA card instead.
It goes without saying there will be no immediate need to upgrade a 2600k or 2500k either.
Of course that's assuming the CPU is set up for a decent over clock and the PC is primarily being used for gaming.







