Overclocking a GPU is rediculously easy, and in my opinion the Toxic cards are overpriced and not worth it. The 5850 is a great card though.
What is your resolution, motherboard, ram, and CPU? A 5850 might be overkill for you, we can determin that for you.
i posted my specs above, but i did have one question, how far off is the 6000 series for the 5000's to take a price plunge?
p.s. my first system build