For an AMD build the CPU/GPU combo is great. The MOBO seems fine and is AM3+ which will help with future upgrades. IMO AMD is not really looking too good these days including their new FX series as an i5 beats them out at gaming.
Your PSU will work well with a single GPU system, not so sure about crossfire (this topic is debated alot over forums) IMO it will just be fine for single gpu with some OC headroom.
btw, good thing, going with a good brand PSU, I learned the hard way as just recently my PSU shorted out and took the Video card with it.
I would go with the latter. There aren't going to be games for a while that *need* six cores, so I would go with the x4 and a better GPU. Then, you have the descendants of bulldozer as an upgrade path a few years from now.
If you cheap out on the video card, you are soon going to have to spend MORE money to get the performance you want. And at a budget of $600, there really isnt any reason to buy any AMD combo, unless you are absolutely trying to cheap out. A Core i3-2100 has close performance to an X6 or higher in most every gaming scenario, and an i5 2500k blows it out of the water on pretty much everything. You are going to have to upgrade your video card before you have to upgrade your cpu if you get the 2500K, that is not the case if you buy an X4 or X6 - you are going to be upgrading both. Considering the 2500K can get to 4.5 on air and an X6 only to 4 if you are lucky. So you have a choice, spend a little more on an Intel 2500K now, and just buy a replacement gpu or sli later, or go cheaper for the x6 and have to spend even more money. Spending 460 just doesn't make sense if you were going to spend $600. If you said your budget was under $500, it would be a different story.
Is $655 (under $600 if you look for rebates and some coupon codes) but would be better in the long run. Overclocking is really kind of pointless at this junction imho. Most games are limited by GPU power with the newer cpus, not by the CPU itself. You should be able to get to 4.5Ghz if you really want to, and there are plenty of forum posts on lots of websites that tell you how.
The chances AMD coming out with anything useful for AM3+ is probably 10% in the next 2 years. Granted, Intel is going to have 4 new sockets by then, but when was the last time there was a "drop in" cpu that gave measurably better performance at a cheaper price? Recent history has shown that is it rarely that cost effective, it is more convenience than anything else.