Okay, I need you to listen carefully. I have done IMMENSE research into this subject for myself.
Say no to the MSI 5850 Twin Frozr II. It has problems going over 1.15v and the cooler isn't much better than reference making it useless.
Speaking of reference, this is what you need. You need to get a reference card because the voltage can be tweaked on them, which is important if you are serious about overclocking. Despite some claims, the reference cooler is very good, especially in a good case, and will perform perfectly for your uses, even heavily overclocked.
Say no to aftermarket coolers. None of them properly cool the VRM and when the voltage is changed, even a little, the VRM will throw out a lot of heat and will overheat on most aftermarket coolers. The Vapor-X cooler will do fine for the VRM cooling, but those are non-reference cards which do not support voltage tweaking. A good example:
http://www.legionhardware.com/articles_pages/prolimatech_mk_13,3.html
As you can see, the VRM is a full 35c hotter than reference when overclocked with the aftermarket cooler. The problem with these aftermarket coolers is that they tend to be universal meaning they provide little to no cooling for the memory or other parts of the GPU except the actual chip.
As for the reference card in question, I highly recommend an Asus 5850 reference card. It will cool fine and let you change the voltages out of the box. If you need to get another brand, or want a better warranty, then you will need to flash the BIOS of the card to the Asus BIOS to allow voltage changing. If you flash a non-reference card it will be bricked and you will have a $300+ paper weight. A good guide:
http://www.overclock.net/ati/671675-how-flash-your-5850-bios.html
All the same applies to the 5870.
note that when overclocked to a 1GHz core, you have reached the height that the 5870 can overclock to as well. At that point, the only difference is that the 5870 has 160 more stream processors, or 10% more, meaning it can only be 10% faster in theory, but may fall short of that 10% because of scaling. You need to ask yourself if 10% is worth $100+ more.