easiest way to do this is to get some RAM that will handle 1:1 overclock at 4ghz without overclocking that ram as well.
e8400 is 9 multiplier and 1333mhz fsb (or 333mhz)
Minimum FSB for 4ghz would be ~1778mhz (or 444mhz)
for ram that will run syncronously would be about half the FSB (double clock frequency) ~888mhz. So youd be best with a DDR2 900mhz or 1066mhz part.
Lower latency is better and there are alot of good 2x1gb parts out there. Id be looking at Crucial, Patriot, OCZ or Geilhttp://www.newegg.com/Product/Product.aspx?Item=N82E16820148069