If using a laptop live for pianos be careful with some problems:
- vibrations; can cause a hard disk failure or a freeze during a performance;
- reliability; buy top of the line laptop with good I/O options (Firewire + USB); a lot of musicians use MAc's but a high end PC made for gaming is also a cheaper option;
- tweaking the OS specifically for audio; forget internet, games, lots of processes running, etc.. Create a solid platform that you can rely on; use this laptop only for playing;
- reading piano libraries; use two separate drives, one for OS and one for samples. If you can go with two SSD, they increase your laptop performance and speed and keep the heat inside way down;
- use a good external audio device with balanced outputs;
Finally... Be prepared for the worst and have a line out from your keyboard to the mixer.
And have fun