I can already see this post having many varied and different opinions.
On the one hand, I agree that having a laptop on stage could be a killer - we practiced this and we had problems. Whether it be mac or pc, there were just too many variable to be reliable. True, madonna (curse me for using her as an example) went on tour with just two computers but what a pain to accomplish.
Sampling and playing loops live isn't too hard but you have to know your equipment. My wife and I do this on Tritons (a studio and an extreme). One of our techniques is to hold a mic to a member of the audience and sample them straight into the keyboard. During the next song my wife cuts and adds effects for the following song. This wouldn't serve your purposes as it does take some time to accomplish.
I'm thinking sampling isn't necessarily what you are looking for. Most keyboards now (arrangers and workstations - not synths) have arpeggiatiors. You can take a sound, pre load or even find it on the spot, arpeggiate that sound to just keep playing until you hit the off button. You can arpeggiate samples as well. Again, you have to learn how to work your keyboard - not learn to play it like an instrument but learn how it works (it's functions) learn its programming. Korg has a function called Realtime Pattern Play Record or RPPR. Yamaha has something similar though I don't know what it's called and I think Roland has something as well. Basically you play a phrase and loop it and assign that pattern to one key on the keyboard. When you are ready to play that pattern back, you just hit the one key you assigned. You can do this multiple times and play back as many looped patterns as you can press - even more if you program for continuous play. You can do the same with samples on the same keyboard but you have to do a bit more programming.
As Sysryn mentioned, tell us what kind of music you are playing and we can try to help narrow this down a bit. I will say that arrangers and workstations are expensive but very powerful. My wife and I are the band. We program all our music and just hit play for the show. I play a live bass and she will play lead sounds while the keyboards play the beats and main rhythms. Our keyboards control effect changes to my bass, run the stage lights, change voice effects, trigger samples, etc. in perfect time. This can be done with one keyboard but we use two for more sound and more programming. Point is, when you do purchase any music instrument, you should always buy with room to expand as you learn. You may not NEED a keyboard at all. The Korg Electribe and Kaoss Pads and other DJ equipment might suit what you are looking for (I'm not pushing Korg either, just using them as examples). Again, give us a little more info so we can try to help you the best we can!!
