Actually atmosphere is a rompler. It won't tax your machine as much as some of the other modelling synths out there. You can also use just a single voice patch, as opposed to 2 from their selection, and not use any of their reverb as opposed to a main bussed reverb.
You have to watch out for some laptops they are selling now at really cheap prices. Though they'll work ok if you want to do some low powered soft synths. Push it a little more and it won't work, degrading the audio with clicks and other stuff. Having said that most laptops will smoke whatever laptops that came out in 2000, but the synths themselves have become a little bit more resource heavy since then.
If you're one for carrying gear, try out some other soundcards than the onboard one, especially those with multi outputs. You'd be surprised what sticking a pedal into the signal chain will do for the sound. That way you can have the laptop play clean backing/samples, and then you can noodle a little elsewhere.
So minimum that would be good would be core2duo 1.8 and above on 2 GB ram, plus no rubbish like in built cam, at least 2 usb ports that are not physically in the same position, firewire, either a pcmcia or expresscard slot. Either a motu ultralite, tc konnekt 8/live, focusrite saffire, firewire 410. But as usual there will be people with niggling issues on all these interfaces. I think the cheapest and oldest card on that list is the firewire 410, so you can chuck that at people if they heckle you in gigs.