If it's nothing too intense, go for some DDR2-800 RAM first. The 1066 won't give you any noticeable increase in performance and will be more expensive for an older generation product. You may even be able to get a dedicated GPU with the saved money.
I know the 4890 was a popular GPU, but I'm not familiar with the performance of AMDs video cards.
A 9800GT would probably work well given that your games aren't extremely intense. That or something along those lines from NVidia would work well for what you play now, but probably not for much more recent games.