I would spend the money now on the faster CPU, upgrading laptop components is not without issues due to how crowded the components are, and purchasing the i5 unless you do it years and years from now will cost more than just buying a laptop with it.
tell me what about upgrading to max RAM of 8Gb (4GB x2) and installing a SSD drive in the laptop ?
will these two also be difficult to do in a given laptop?
Drive and RAM are much easier to upgrade, and really the only things that are made to be more or less easy to replace.
Just one thing, keep in mind when you purchase a laptop with say 2 gig and want to upgrade, check if the 2 gig comes on one chip or 2. Most laptops only have 2 memory slots, so if you have two 1 gig chips, you need to dump them to upgrade to 4. If you have one 2 gig chip, just purchase one more.
I have some 5-6 year old P4 Mobile laptops that are running Windows 7 on 1gig of RAM, so an i3 should be good for just about anything for 3 years, and lower end (future lower end that is) for another 3.