I would get the K version too. I think the budget is very realistic, and you can get a very good motherboard in that budget. I don't know the whole advantages of the very top end motherboards, maybe someone else is more informed on this. If you are from the US, and have Microcenter, even better for the budget, but i am not sure if you are.
But, if you now have DDR2 memory, they won't work on the new system, and you have to replace those too. Getting DDR3 will affect your budget, but just a little , because they are very cheap. I'm just 99% sure of this, because there may be motherboards with this standard too, but i don't think they are recommended.
I think you can get a very nice Gigabyte motherboard in your budget. Of course Asus, MSI, ASRock and EVGA are or can be just as good or better. Asus and EVGA especially for allround quality, but the other 2 ones are close or very close on this too. MSI especially is marketed with very high quality components, and i think they really are. AsRock has also taken huge steps lately, thay were pretty good in the beggining too. (I for ex. have a very nice AsRock one. It's very stable, it just doesn't "move", or any kind of problem. The same goes for my old Gigabyte one(other generationof cpus), which was really cheap. Of course there are other factors to take into consideration, but stability is a good one. Maybe others can help you with this.
I would reccomend you a Gigabyte model, but i am not really up-to-date with the current models.
I would look at the 4670K also, since it's newer, not neccessarily or clearly better, mainly because it heats up more during overclocking. But it has more and newer instructions