Very few games make use of hyperthreading, so the i7 isn't going to provide much benefit over an i5.
The Samsung Evo might be a better fit.
Might be worth getting a 250GB class SSD and a quiet/slow HDD for the rest. If you drop to 128GB, the 3-bit SSDs drop a lot of random write speed...so, you might consider the 840 Pro.
PSU: If you upgrade your GPU, it may use two PEG connectors. Make sure your PSU has 'em. That said, your stressed load power with the 6850 is ~250W. If you move up to even a 200W class graphics card, that just raises the max stressed load to 320W..and probably <280W gaming.
Fans: better off having multiple slow moving fans.