I have an emachines m6805 that has an ATI 9600, and an athlon 64 3000+ I get like 1.5 hours of gaming out of it. After taking into account that the 3000+ is on the old 0.13µm, I'd say you'll probably get a good 2 hours of of Half-Life 2.
You should look into getting the ATI
Omega Drivers, try setting the ATI x700's clock speed lower. That might give you a few more minutes of play time. I've done this with my Radeon 9600, and I find it helps a little. If you can hard set the processor speed lower, that will give you more time as well. you probably could get by setting your Pentium M at like 1.2 - 1.4GHz on HL2 rather than running it at the full 1.86Ghz. One of the biggest drains on battery power is the LCD screen itself, lowering the LCD brightness will add a good chunk of time as well, especially if you have one of the larger 15.4"-17" screens running about now-a-days. when all is said and done you might only squeeze out 15min, or you might squeeze out 60min I can't realy say.
Anything I said is not my professional opinion if what you do voids your warranty, ruins your laptop I am not responsible for. I am also not not responsible for any damage caused by the modifications suggested. You proceed at you own risk. Hope this helps.
-manno