Best way to improve on build?

Hello, I built my first homebuilt system back in July, and I am wondering what would be the best way to give my system a little more performance . The specs are as follows:

CPU: Intel Core I-5 3570K
MOBO: MSI Z77A G-43
HDD: Hitachi Deskstar 2TB 64MB
RAM: Corsair Vengeance (2x4GB) 1866mhz
PSU: Thermaltake TR2 600w
GPU: MSI Twin Frozr 7870
BD-ROM: LG Black 14X BD-R...
Case: Cooler Master Half 912

(All the components are running on stock settings and stock cooling is present throughout the entire system.)

Should I invest in an ssd now? If so which one would be reliable and economical?

Would the stock cooling be sufficient for ample overclocking, or would I need to invest in aftermarket cooling?
I do live in a high dust environment with the computer situated several inches above medium carpet though cleaning out the computer monthly seems to be doing fairly well.

I use the computer primarily for Skyrim(heavily modded), Bf3, and minecraft(sometimes as server while playing Bf3 or Skyrim).

Please tell me if this post is off-topic for this category?