Your expectations are possibly a little high. I have almost exactly the same specs on my laptop (2GHz dual core, 4GB RAM, Geforce 9600M) and on Audiosurf I can't use maximum settings at 1280x800 or I get jumps. On STALKER I have to turn the settings down somewhat for smooth gameplay. Source games tend to run really well. I think the real problem is overheating. Nvidia cards tend to get hot, and in a laptop this is made worse. If I'm in a cold room I tend to get a few more FPS for longer periods; performance deteriorates as the card warms up. There's not much you can do to get better performance apart from turning down graphics settings, messing about with background processes or just getting a more powerful computer.
Dont be afraid of opening up your lappy and cleaning out the dust from the fan (blow it outwards from the inside, not inwards from the outside) and make sure all airflow vents on the bottom are clear of dust.
I updated my lappy from a T5550 (1.83ghz) to a T9300 (2.5 ghz) without any additional cooling or even applying more conductive past.
My specs are pretty much the same as yours through.
Intel Core 2 Duo T9300: 2.5ghz
Nvidia 9600m
Only 3 gigs of ram though, I need to get a second 2gig stick
Running Vista Home Premium
And I am currently playing Arkham Asylum at 1366x768 with no Anti-aliasing, Ambiant Occlusion and Motion Blur, everything else is maxed. Borderlands runs even better, as does Bioshock. All 3 are Unreal engine games. Both Oblivion engine games; Oblivion and Fallout 3 run well. All Source games run spiffy, would like to give Crysis a go, I havent tried Dragon Age as Im pretty sure that will be the hardcap where my lappy cant handle it, maybe with more ram itll be alright.