Upgrade or scrap and start over?

I have been using this rig for a few years and I can't decide if I should put more money into it or scrap the thing altogether and start from scratch. I bought it as a prebuilt and have been upgrading for a while. I am looking to be able to play BF3/Skyrim/Far Cry 3 on High to Ultra at 1080p. Right now I can barely play at medium. My budget is about $500. Overclocking is also a factor. Any ideas?

My current setup:

CPU: AMD Phenom II X3 @ 3.2GHz unlocked to X4

RAM: 2x2gb DDR3 1333

Mobo: Biostar A870U3

GPU: nVidia GeForce 550 ti

PSU: 550w

HDD: 233gb 7200rpm Hitachi, came with the prebuilt.

I have a case/optical drive/all peripherals. I will be getting a new HDD and some more RAM regardless.

Thanks in advance.