Please don't listen to mafisometal.
First of all, it will not bottleneck. It would hardly bottleneck cards like GTX 580.
Secondly, he doesn't need 500 watt unit for his setup. 250 watt will be plenty.
As for going with HD 6670: could you list your motherboard model and size of your case (height, width and length)? You know, the card might be too big if you have a small case (in which case I'll recommend a card that will fit) or your motherboard may not have a slot that fits.
A lot of motherboards can bottleneck because of simple build quality...all i was pointing out was that the PCI controller strings are connected to the i3 processor...The i3 gives the direction and the PCI slot will receive and send a feedback to the CPU...thats where the controller works from, it will re-inerpret the pixel count back to the GPU and which the CPU will render out what's been told. So an i3 chip can bottleneck higher performance cards. the i3 has a higher latency controller than the i5 and i7. BUT its not drastic where its insanely noticeable.
Anyways, 500W PSU is just for safe measure, sometimes Power Supplies don't give as they state, most PSU will only give back 80% power that is stated on the specs. I.E. 500W Theoretical, 400W Realistic. This means the 250W PSU would most likely limit at 175W and then anything past that will stress the PSU and shorten its lifespan. Count up the power consumption by the processor(70W), HDD/SDD (25W TDP), Disc Drive (10-20W), Graphics Card (25W-70W on a passive Graphics Card, if you run anything with the need of a 6 or 8 pin connector then we go to 100W+).