I'm with carnage here. The 6670 recommends a 400W psu. The biggest question is how many Amps are on the 12V.
I think you are just squeeking buy on the power side of things, if the gpu starts being under powered i think it will lag.
What program are you rendering with? If it takes advantage of multi-threading on lots of cores, an x6 may be an option. But unless you can use all 6 cores, its wasted money.
But in reality, i think you need a better gpu and with that, means a bigger psu or 600W ish. This however is over $200.
oh Forget getting ram....... 8Gig of ram won't help you, stick with 4Gig, the timings are crappy but it it 1333 ddr3... Why do people not remember it's DOUBLE data rate, so 669 x 2 ~ 1333. A quick google search on the board confirms, it's only DDR3 capible.
What are your temperatures... If youre getting to hot, you could be hitting throttling, and an inexpensive cooler could be a great investment.
In any event, you have 2 real bottlenecks in your system.. GPU and PSU, which will cost bout $300 (in the US/Cda) to upgrade.
Do you have an online store to let us look at prices over there?
a good store is mindfactory.de
temps are: MB at 51 celsius, CPU at 35C and GPU at 45C, some programs on, drawing in photoshop a bit and minor stuff.
I'm rendering with Sony Vegas, i'm not sure if it uses multicore. But i know that GPU is very important in video rendering as well.