The recommended specs on the box are 1ghz CPU, 256mb mem, and a 128mb Directx 9 compatiable video card. So basically, still life will run fine on just about any PC that has a geforce fx or higher vid card. Nothing tough there for a desktop but a 128mb of dedicated video memory and a newish GPU on a laptop will cost you. I'm not sure you will find a laptop that packs enough of a wallop for current and future gaming in your price range. On the other hand, if you just want to play some of the older classics then a $600 lappy should handle them fine.
On the desktop end, $600 will get you a descent gaming setup. AMD 3200+ ($150), ATI x1600xt or Nvidia 7600gt ($175), 1gig memory ($75), case ($50), Power supply ($50), 80gig drive ($50), DVD-burner ($50). Plus, cannibalizing your current system for parts for you new rig and eBaying what you don’t need will drastically help you afford more power.
It all depends on how much you want to spend, how mobile you want it to be, and how future proof you want to be.
*edit: as for sitting in your car and killing an hour... I recommend getting a DS.
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...bysmitty