I've got the exact same spec lappy as you, though my resolution might be different - mine's 1280x800.
At that res, I can play every game I own comfortably, most with high to max settings.
The 8600mGT overclocks nicely, and mine overclocks fantastically (I made a thread showing benchmarks etc in the graphic card forum).
Source games you can expect to max out. Bioshock can be maxed when the card is overclocked, otherwise the odd setting needs lowering.
UT3 runs at high settings, with the two sliders set to 4 out of 5.
CS:S stress test at max settings, 4aa, 8af got 96fps (~120 overclocked, iirc).
*edit*
Just re-read your thread.
TBH, I reckon the card will struggle at 1680x1050 - more due to the 128bit memory bus than anything else. It'll still handle most games on medium settings, but I'd be tempted to see how decent the screen is at scaling 1280x800 - there's a chance it won't look like mud
Word of advice - if you do try overclocking the 8600mgt, memory clocks give the highest gains. The gddr is already restrained by the 128bit bus, so anything you chuck at the ram results in fairly impressive gains. As an indication, mine's running 670 on the core, 520(1040) on the ram
I haven't really played any other demanding games, as I've been playing pennyarcade / casual games recently. It's a decent card though
Use rivatuner to overclock the card - you'll then be able to set hotkey profiles, so you can quickly switch to higher clocks when the machine's plugged in, and default clocks when on battery power.