Have you gotten everything you need from this job, professionally, financially, and personally? Are you satisfied this move is the best choice for you to grow professionally?
If you want this other position, give your current employer proper notice, and work with them to make the transition as smooth as possible. You can even discuss the possibility of being available as a consultant for a while if they need.
On the other hand, you might look over the offer and decide the trade-offs aren't worth it. Sometimes, the increased money is accompanied by increased stress...longer commutes, longer hours, more demands on your personal life (e.g., being on call 24/7). An interesting and rewarding job that pays less might be the better option then.
Chances are, however, you're intrigued by a new challenge, a new beginning, and better pay. Why not take the leap?
Remember, in the end, you are replaceable. It's part of business. People get sick, hit by buses...or better offers. Leave gracefully, don't burn any bridges, and you can continue to stay friends with those co-workers with whom you are close.
Best wishes on your decision. In this economy, it's rare to have such a choice to make!