I used and still use many laptops and even netbooks to host Arturia instruments (MgModular). A dual core CPU with 2+ GB RAM will have plenty of power, together with an ASIO audio interface (USB or firewire).
There is only one 'trap': some USB interfaces don't like some laptop chipsets. This is rather rare, but if it happens, it is unusable because of bad performance. I know of only one such combination, the Acer 7730 laptop together with NI AK1 interface. Using the laptop with different interfaces and using the AK1 with different laptops is no problem. But I think if you buy relatively modern equipment, there won't be any 'bad combinations' like I mentioned. My AK1 is a USB 1 type interface, and obviously badly constructed (also mechanically with bad switches and sockets).
After buying the laptop, you can test the audio interface first, and bring it back to the dealer if something is wrong (which is rather unlikely with USB2 type interfaces or firewire). Fast USB2 interfaces started appearing 2008. I have two of them from M-Audio (FastTrack and FastTrackUltra), they are very good and reliable.
