Keep in mind that upgrading to 8gb means pulling the two existing 2gb DIMMs and replacing them with two 4gb DIMMs. In other words you can not get to 8gb by adding to the existing RAM. A 8gb kit will set you back about $450-550 US, I just paid about $460 from OWC. Look at your usage to decide if the extra RAM is worth the cost. I went with the MBP 2.53 rather then a 2.8 using the money I saved to pay for the extra RAM. The main use of the MBP will is working with RAW files in LR/PS. For me the extra RAM gets me more then the faster CPU/GPU of the higher end MBP.
Installing RAM or a new HD is pretty easy if you have the right tools. OWC has some very good videos on how to make these upgrades. My one word of advice would be to make sure you seat the RAM fully in its slot. I had some errors after installing my RAM. After opening the MBP back up and reseating the RAM the errors went away.