I am currently running Digidesign Velvet on a Mac Book Pro that has a 2.2 Ghz Intel Core Duo processor and 4 GB of RAM. I run the software in Apple Logic most of the time and will sometimes rout it to Pro Tools LE 7.4 if I so desire. I am able to run the software smoothly and stably within this configuration and have never had any problems in this category at all. As long as you have an adequate system to run your sequencer on, I can't imagine that you would have much trouble running Digidesign Velvet, but it really depends on what you are running...
I've been using Digidesign Velvet for a few months and it has become one of my new favorite electronic piano virtual instruments. It has a very sweet sound and sounds very similar to the real thing. I like the fact that you get so many different parameters to work with as it really extends the palette of sounds that this software has to offer. The price is very reasonable, so if you are looking for a great sounding and versatile virtual instrument when it comes to the electronic piano, Digidesign Velvet is a must have.