If you are not getting a new CPU, it makes no sense to replace the motherboard. There will be no real performance increase from it. Besides, changing the motherboard requires reinstalling Windows in most cases.
Your current motherboard supports the video card you want. Go ahead and get it, but invest in a nice 500W brand-name PSU too. OCZ is a good choice - as are Seasonic and Enermax.
You have good RAM and plenty of it. Leave that.
As an aside, that 266MHz speed is the unmultiplied speed. Multiply that number x 4 for the effective speed of your RAM - 1330MHz.