Yes it comes with a molex to PCIe adapter, mine came with it and it depends on the brand, but I assume most would.
I don't know about that PSU, I always avoid them when I make rigs for my friends. I have so far stuck with OCZ, Corsair and Antec and none of my friends or myself have had an issue with our rigs. However, choice is yours, as you can get a fully modular 550W PSU that can push the GTX670 without an issue and should last long time. So go to newegg and search better PSUs.
i plan on upgrading to an i5, or i7 and getting more ram so should i get more than 500W?
You should def get a new PSU and go from there and a 650W PSU is more than enough for your rig but I'd get a 850W PSU if you are planning to add another GPU later on. Lots of things are on sale so go to www.newegg.com and search around. All the best!