Do you need an OS?
If you don't this is the best build you'll find for that price: http://pcpartpicker.com/p/dy9WJx
Or if you want an AMD build, here's the best: http://pcpartpicker.com/p/FdzMMp It costs 30 bucks more but has a lesser gpu but you can OC this build. I suggest you just go with the intel build because it's more upgrade proof and has a better gpu.
If you do need an os look at this: http://pcpartpicker.com/p/ZFqkHx
Both the non os builds will give you med-high on all games. However the Build with os included will give you low-med on all games.
Use something similar to this, but edit it to your needs. PCPartPicker is a great website to do this, and you can compare prices between merchants.
I guess AMD and Runescape don't go together well, so that's why I would stick with the Intel processor (Stronger but less cores). You can always swap out that processor/motherboard with whatever you think suits your needs.
You will need to install the OS via USB. Or if you use cds/dvds you can add an optical drive. Should max out RS and play other, more demanding games around medium settings.