Apple charges you 90 dollars for 4GB, you can buy and install it for a lot less and save yourself some cash.
RAM modules found here.
I recommend the first one on the list, Crucial.
With the extra cash that you saved on the RAM you can put that towards the HDD, which are faster than what they offer and have large storage capacities. I recommend these two.
Forgot the how to guide to install the stuff. That can be found here.
I am going to buy the basic model of 13 inch macbook pro with 2 gigs of ram and 160 gigs hard drive. About the ram upgrade, can I just add a 2 gigs ram to make it all together 4 gigs or do I have to buy a new 4 gigs one.
Same question about the hard drive.