Now, on my XPS Gen 2 I went from 2.13Ghz / 6800 Ultra to a 2.26Ghz (pin mod) / 7800 GTX. Normally, one or the other wouldn't be worth it on it's own. But doing both together for only $400 dollars total, it was a 25% speed increase (with NO overclocking) for only a 17% total cost increase (including the original cost of the laptop). That's the exact opposite direction typical diminishing returns work. Heck, that upgrade would cost you $650 dollars if you ordered it from Dell, still over $400 dollars with a 33% off coupon, before shipping and taxes.
And, with this mod, I now have parts available to mod an i9300 up to XPS Gen 2 specs. That should cost a total of approximately $1200 dollars (i9300 with 33% coupon plus 9 cell batt plus 130W power supply). With less than 4k invested, you now can have an XPS Gen 2 class AND an XPS m170 class laptop, that can shred just about every game out there, and will last a long time. Heck, if I had been smarter at the beginning, I could have bought a i9300 instead of an XPS Gen 2 and saved even more money!
Finally, about undervolting. If you're planning on keeping stock (other than pinmodding), undervolting is a no-loss deal. It'll throttle up when you need it, and even at full throttle, you'll still shed at least 15C, more if you AS5. Battery life? Well, a little, but the heat thing is a big deal, even if you stay in clocking spec. More than worth it (it's free)!
I was kind of King Kong and did everything at once (Pin mod, 7800 GTX, undervolt). Spent one evening on it, methodical like, but now, I have a rock stable laptop that kicks butt...trust me, you think FPSes can bring a computer down, try Lock On Modern Air Combat at 1900x1200 with all the bells and whistles.
Anyway, with my pinmod 1.7->2.26 I can undervolt to .7/1.262 and my CPU never goes above 51C (using Hot CPU Tester Pro) and my GPU never climbs above 87C (using ATItool with A04 XPS BIOS).
Good luck, and enjoy!
PS I treat this like a hobby anyway. And it's a hell of a lot cheaper than my last hobby...hot rodding musclecars. You ever cost out what it takes to make a 71 Mach 1 a show car with a 400+ motor???? This is cheap (and less greasy)!