Unless you don't use your G74 at high loads, I would not use a 120W power supply on the G74. A quick survey of G74 component power ratings shows the minimum wattage that the G74 needs at "full throttle":
Core i7 2630QM/2670QM Processor TDP (max)= 45W
NVIDIA GTX 560M (max) = 75W
Chimei Innolux 17.3 LED (N173HGE-L11/L21) = 9.3W
Seagate Momentus 7200.4 500GB 7200RPM (x 2) = 3.1W each x 2 = 6.2W
Intel HM65 Chipset TDP = 3.9W
Total ~ 140W. This doesn't even account for the power required by the motherboard! In desktops you always bank on another 75W for just the motherboard (when you're buying a PSU for example), but since the G74's is smaller it'll probably be closer to half that, say 35W. So thats 175 watts right there, that you're already trying to pull from the stock 150W supply. It gets very hot because 1) you're pulling near its maximum capacity, and 2) the low and high end of a power supply's capacity is where efficiency is smallest, and therefore more wattage from the wall is converted into heat.
Go with a 180W unless you travel: stick with the 150W then. A 120W supply would be run over-capacity by alot, it would fail faster, plus it would be horribly inefficient and make more heat than even your 150W!
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G74SX-CST1-CBIL, i7 2630QM 2GHz
32GB DDR3 RAM @1333MHz
GTX560M 3GB DDR5 (192 bit)
17.3" LED 1920x1080
Sentelic TP, BIOS 203
Debian Linux Wheezy (Testing) Kernel 3.2, NVIDIA 295.40
