Upgrading...it depends on your budget and what you want to do. If you don't tell us, we will assume that it's gaming.
For gaming, graphics matters most, then your CPU, then your RAM. But the Power Supply (PSU) is very important to make sure you safely and reliably power your whole system. Also, your HDD is probably significantly slower than newer HDDs. If you'd like to check out your HDD performance, download HDTach: http://www.simplisoftware.com/Public/index.php?request=HdTach
A modern HDD should get at least 100MB/s throughput. I'm guessing yours will be in the 50MB/s range. That directly affects load times and startup. It will have a negligible impact on gaming.
You're going to be bottlenecked by a dual core CPU on same games no matter what. You can get a lot better performance if you can overclock the Athlon II x2 up to 3.5GHz or so. If you want to do that, that could deserve it's own thread. Make sure you watch your temps.
Right now, your graphics card is not a gaming card. It's more like a low-level media center card for video playback, etc. What games do you play? If you barely play games, then your current card is fine. If you want to play Crysis II, then you should get the best card you can afford.
What is your motherboard model? What is your PSU model? What resolution is your monitor?
If your motherboard is AM3+ (not AM3, AM3+) compatible, that opens up some options (down the road especially).
So we'll need all the questions I just asked answered as well as your budget.