I agree with the above (mostly):
1. When a Power Supply is rated at "300W" that means the TOTAL power on all of the Voltage outputs. Not only is that power supply insufficient in raw power (a GTX670 would need a 550W or greater) but you also need to verify the power output on the +12V rail. So you need to ensure:
a) the overall Wattage is high enough (i.e. 550W or greater)
b) the +12V rail Amperage is sufficient (the easiest way is to verify it has the proper 6-pin and/or 8-pin connections for the graphics card), and
c) the needed 6-pin and/or 8-pin connections exist
2. CPU and bottlenecking:
This is somewhat complicated, however by my (very rough) estimate your CPU when paired with a GTX670 would achieve between 50% and 75% the frame rate of the GTX670 when paired with an i5-3570K CPU.
I see THREE main options for you:
#1 - Buy a GTX670 and put it a new build later (not recommended).
#2 - Buy a better CPU and GTX670 (not recommended).
#3 - Buy a graphics card like the HD7850 2GB that isn't as bottlenecked (recommended).
***I want to be perfectly clear, you WILL achieve higher frame rates in some games if you buy the GTX670 versus the HD7850. The problem is that the difference likely won't be huge so the best VALUE is likely to buy no more than an HD7950.
4. You'll still have a pretty nice gaming system with "only" an HD7850 2GB card. Here's my recommended cards:
Power Supply (example, look for SALES too at NCIX, Tigerdirect etc. quality brand Antec, Corsair other):http://www.newegg.com/Product/Product.aspx?Item=N82E16817139005
- recommend one of the above cards or similar (get at least 2GB of VRAM, not 1GB)
- 550W PSU minimum (ensure proper 6-pin/8-pin connections)
- CPU bottlenecking amount varies by game