In general, you should consider laptops to be WYSIWYG. They may not always look it, but they are very finely calibrated machines. The power brick is made specifically to be able to power the hardware that came with the unit, the cooling system is designed to be able to handle the waste heat generated by the hardware that came with the unit, and pretty much everything else is calibrated specifically for the hardware it came with.

AFAIK:

1) The CPU *should* be upgradeable to an Ivy Bridge. They are socketed the same (FCPGA988), have the same architecture (an ivy is just a sandy on the new 22nm litho), and the same form factor and size (rPGA988B). But, it is very important that you put in an Ivy that has the same TDP (in Watts) as the 2670 (45 W). If you put a 55W TDP Ivy in there, you will a) stress your power supply, b) stress your VRMs on the mainboard (like an overclock would) and c) stress your G74's cooling system. And there is the risk that ASUS' BIOS for the G74 may not be built with the future in mind...and so may not recognize an Ivy. That said, however, I haved read posts from G74 owners on iother forums who have stuck in the 2960XM (the fastest mobile sandy, and a 55W TDP) into the G74 and had it recognized and work well. Just be sure to buy a beefier power adaptor (like a 180W at least) and run the machine in a cool environment, and don't try to win benchmark contests by stressing the mainboards VRMs to deliver power to all 8 threads.

And remember this upgrade voids your warranty.

2) RAM is upgradeable to 32GB @1333MHz. You can put in 2x4GB=8GB of 1600MHz or even 1866MHz clocked stuff, but the G74 will not run the RAM at these higher clocks with all four banks filled with 4x4GB or even 4x8GB (it will downclock back to 1333). Not sure about 2x8GB of 1600 stuff...look around on the forums and see if someone has done this successfully. Unless you need more physical RAM an upgrade of just the clock speed from 1333 to 1600 is almost pointless, since you won't even detect any speed increase in day-2-day apps. An SSD is a far, far, far superior upgrade that will leave a lasting impact on you!

3) The GPU is soldered to the mainboard, so the G74 is essentially stuck with the 560M. I wouldn't even know how to begin to attempt to upgrade it: desoldering the chip, ensuring every pin on the new GPU matches the 560M's pin array, etc. Brutal.