It really depends on what you use your computer for. People who just surf the internet and listen to music might be fine with a single core. Most users, even gamers, will be happy with a dual core. Heavy multi-taskers/encoders/'hardcore' gamers etc will want a quad core.
Here is an article from Tom's on that subject.http://www.tomshardware.com/reviews/multi-core-cpu,2280.html
And from the CPUs you listed the best for the price is a either the i7 860 or 920. You might want to look into other options as well (unless you aren't looking to purchase) such as i5 and Core2Duo, or on the AMD side, Phenom II and Athlon II.