how much should I overclock

I think koala's link is broken (at least it isn't working in my browser). Try this one. I'd also add that you are trying for some very large overclocks. You might be able to overclock your GPU that much with better cooling but you could also damage it. And your RAM overclock to 900MHz is extremely optimistic. I've seen people occasionally get away with large RAM overclocks only to find that it damaged the RAM in the long run. The overclock worked for a month or two and then they found that they had to reduce the RAM clock rate more and more to get rid of artifacts. Eventually they end up with RAM that can't even run at stock speeds anymore. The video card you're looking at does not have RAM heatsinks so your RAM is going to get very hot. You can watch the GPU temperature to make sure you're not destroying it but you can't do that with the RAM. Be careful!