You can always remove the stuff you installed before you get it repaired and put the original stuff back in
Don't buy a bigger HD or Ram from Apple, go with the bare minimum they sell it at as it will be much cheaper for you to just buy ram & HD locally from a normal PC distributor or reseller.
Edit: The one optional extra I would consider is the Optional 15.4-inch (diagonal) LED-backlit widescreen display with 1680 by 1050 pixels (glossy or antiglare) depending on cost.