Welcome to Tech-Forums, deaydan!!!
The best upgrade option for you, would just be the graphic card and power supply.
Upgrading the processor won't really do you anything because the motherboard probably has a very limited choice for higher processors. Besides, the processors above your's are only a few 100MHz faster, which won't be noticeable.
You have more than enough RAM, even too much RAM. But that's good, you don't have to worry about ever needing anymore RAM for now.
That hard drive is plenty. You would want to upgrade it if you see yourself filling up that TB first. See how much is left before you decide.
So for the power supply, this is good enough for your upgrade, plus future upgrades:
Newegg.com - Antec BP550 Plus 550W Continuous Power ATX12V V2.2 SLI Ready CrossFire Ready Modular Active PFC Power Supply - Power Supplies
Though I'm not sure if your computer will accept a regular ATX sized power supply. I'm checking on that now.
And an HD 4870 is not that much more than the 4850:
Newegg.com - SAPPHIRE 100279-1GL Radeon HD 4870 1GB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - Desktop Graphics / Video Cards
Then just save that extra money for something else.