If you are frequently in more than a few apps at a time I'd recommend the duallie. If you only use a single app at a time such as email, or a browser or burning a CD then the 1.4GHz would be nice.
You can't go wrong with either but you'll notice the difference with a dual processor upgrade if you multitask more often than not.
In fact, I went dual about 5 years ago and I'd not likely ever own a single processor system again (other than a laptop).
YMMV and good luck.