Upgrade pc components to play skyrim ultra

Hi! I want to upgrade my pc to play skyrim heavily moded on best graphics, but I'm also on budget.

My specs are:
Processor: Intel Core 2 Duo E8400 3.00GHz
Motherboard: Asus P5P41D
RAM: 2x2 GB DDR2
Graphic card: Geforce GTX275
Power supply: ATX12V PK-550

What do you think I could leave and what should change?
Thank you very much in advance!