A few questions

I used to be a hardcore D2, Warcraft3 blizzard gamer but have been out of the loop for 3 or 4 years. I recently bought WoW but I know I need to upgrade my system. I own a Sony Vaio PCV-RS300 series. I want to add more RAM and a new graphics card. My RAM is a 256mb stick. I want to bump it up to 512 for now. Is it better to keep one stick in there and add another 256mb stick, or pull that one out and add a single 512mb stick? It has 4 sockets and each one maxes otu at 512. I also considered adding 3 more 256mb sticks. What's the best way to optimize this?

My other question is about a graphics card. Mine is currently a 64mb standard integrated card. I want to add a relatively inexpensive nVidia card like this one>http://www.circuitcity.com/ssm/nVidia-by-EVGA-e-GeForce-8500GT-512MB-DDR2-Graphics-Card-512-P2-N747-LR/sem/rpsm/oid/207231/rpem/ccd/productDetail.do I'm assuming it's compatible. I just uninstall the old drivers, turn the power off, unplug the old one from the AGP, put new one in, and reboot and install the new drivers, correct?