This seems to work.
Sony GRT170 Laptop 2.8 GHz P4, expanded to 1G RAM, 60GB HD, 16" LCD. Add Indigo IO card for ASIO drivers and high quality AD and DA conversions. Yamaha UX16 is my MIDI interface which I use with various keyboards as controllers. As long as you have an AC outlet, you are fine. Don't expect much on the internal battery.
I am running, Forte (Demo), Kontakt, Virtual Guitarist, NI Carbon, X-Phrase. I have not tried HD streaming with Kontakt with this system.
If you are going to carry stuff around, there is no more need for a desktop computer.