RAM is RAM, who makes it doesn't affect anything. You can't put in DDR3 or DDR RAM because they have different amounts of pins. You can put in faster DDR2 RAM but it will be underclocked to what your lowest RAM module's speed is. If you plan on adding more RAM keep in mind you need a x64 bit operating system to go over 3GB.
You can buy more RAM, the only problem is that it will be expensive. The reason being that DDR2 is less common from not being produced as much. 8GB of DDR2 would be around $200 versus the same amount of DDR3 for $80.