My experience was that after being told the price (my MacBook would have cost £70 pound more that way than ordering it online) and complaining about the price difference, they matched the price from the website. In the end, in your example, if they install the 2 x 2GB sticks for the price difference and keep the old 1GB stick in their spare parts box, they still make a lot more profit than if you bought a Mac with 1GB only.
Just reading that you are there for a very short time only: Whether they give you the better price or not depends on whether they want to or not. If they think you don't buy at the higher price, you'll get the lower price. If they think you will buy at the higher price, you won't get the lower price. So don't tell them you are there for five days only because then you have no choice.
Well, going back to what the OP asked, he's wanting to upgrade the HDD and keyboard. So, whether they match the price on a ram upgrade is a moot point.
Can i buy it online and get it next day or in 2 days?
I dont have a US address credit card so i will have to get it with a friend once i am in the US and i am only staying for 5 days before coming back to Costa
Rica
You can get 2nd day shipping, but the thing is, that doesn't take into account build time. I just went to Apple.com, and if you choose the 17" with the high-def screen and a 250gb HDD, the build time is 2-4 days. So, if you were to order it on a Monday, it could easily ship that day, but could also ship as late as Thursday. Then, since you can only get as high as second-day shipping, that puts its arrival date on the following Monday. So, yes, you can order it online once you're in the country, but there's a good chance it won't arrive until after you're gone. It could come in time, and it might not. And their shipping times may change by the time you need to place your order as well.
...or can you go to the Apple store and order it custom through them?
I actually believe you can do this, but it would have the same problems as I mentioned above.
If you really need to order it in the US, and you have a friend here who's going to have to give you access to his CC anyway, I would say your best bet would be to contact your friend before coming to the country and get him/her to go ahead and order it beforehand. Then, you can simply pick it up from him/her once you are in the country. You can try some of the other options above, but you will have a good chance of not getting it in time.