I have a Studio 1555 and it comes with 2x 2GB sticks totaling 4GB, If you were to increase the ram you would have to take the older stuff out. I found some people doing upgrades to 8GB (2x 4GB). Amazon says it is expandable to 8GB (http://www.amazon.com/Dell-Studio-15-6-Inch-Chainlink-Laptop/dp/B002PY7OSA)
Scroll down a bit. So as long as you have a 64bit OS you should be able to utilize 8GB.
If you want to upgrade you need to buy a 8GB DDR2-800 Kit. (Notebook Memory)