1. Is 4GB not enough RAM for you?
2. Upgrading won't affect your video card as it uses a PCI-E slot, that's pretty universal nowadays. However, it will really affect your CPU. You'll have to make sure it's entirely compatible, and it can be quite a hassle to change them over, you need to remount them, apply more thermal paste etc. Quite annoying. Can be done though.
3. It'd be easier to buy a new CPU, as the socket on there is quite old. What is your current CPU?