If you're intending not to lug your keyboard along, you need to have a gig bag of tools that will enable you to play any keyboards. This will indeed include a midi interface, since the keyboard in the studio may not be USB capable. USB midi interfaces are not expensive. I usually have 3 pedals in my bag as well - one for sustain, one for switch, and one expression pedal.
The flexibility of switching patches and playability depends 2 things:
1. The programming of the softsynth itself
2. The controllers the keyboard has (how many faders, knobs, wheels, and whether they are assignable)
The last thing you want to do live is to twiddle around with your notebook's mouse or mouse pad. Omnisphere, as evident from the youtube clip, is very versatile live. Notice Jordan Rudess switching sounds using the Korg Nano. That's call key-switching, one of the many ways of programming for patch/sample change. You don't need that if you're using a keyboard with 88 keys - the switch is usually programmed at the lower octaves. But Jordan is using a 76-key synth action keyboard, hence the Korg Nano to extent the range. You can also program a patch/sound change using the mod wheel - ie by cross-fading. Mod wheel at one end is one patch, at the other end is another patch, anywhere in between is a mix of the 2 patches.
Forget about the Netbook. The only known softsynth/sampler that is known to work with the Netbook currently is Garritan's libraries (Garritan Personal Orchestra) running on his ARIA engine. It's orchestral-based. For most of softsynths that rely on a lot of synthesis, expect CPU and RAM drainage. You would want to get something that's fast and has lots of RAM.
You would also want to dual boot it so one boot is entirely dedicated to playing music. Do a search on the forum on dual boot - it has been discussed in great detail in many previous posts.