General rule is that you need about 400MHz today to see the difference outside of a benchmark. I would say that you would believe that there is no difference between them with that little difference in performance.
Here are the processors Ultra low that might be an option http://ark.intel.com/products/series/28964
You might also be able use an Intel Core 2 Duo but in that case you are going to deal with a lot more heat and shorter battery life.
Here are all of them http://ark.intel.com/products/family/26548
in most cases you are going to see the TDP go up by x3 times.
The higher TDP might increase temps far beyond the what the factory cooling is designed for and cause throttling, lockups, etc... Not worth IMO. Other then that, the 400MHz rule is spot on. If you aren't using the PC for rendering, gaming, encoding, or watching HD movies you might not even notice the upgrade all. If you simply want better performance for daily tasks, office software and web surfing a RAM upgrade makes far more sense.