I don't know how much in-store RAM upgrades will cost.
Hard drive size depends on your needs. The default OS X installation will bring you down to about 40 GB of available space. You can get a good portion of that back if you don't plan on using GarageBand and iDVD and delete the programs and their support files from your computer. Getting rid of extra printer drivers and foreign languages will also save you some.
Why don't you order online to get the exact configuration you want?