Overclocking a 2500k/2600k will not force your RAM to be overclocked, as you use turbo multiplier rather than FSB.
RAM speed makes next to no difference in gaming/real world performance, and really only shows in benchmarks and even then only insignificantly. 1333mhz / 1600mhz are where its at price wise. 2100mhz is really only gimicky because of slow timings and high price.
Either of the following 1600mhz Ripjaw X's would be ideal, with one cheaper due to slacker timings (CL9 vs CL8, CL8 is better). Basically no difference in performance, just thought i'd link both in case you're following a colour scheme. http://www.newegg.com/Product/Product.aspx?Item=N82E16820231427&cm_re=ripjaw_X-_-20-231-427-_-Producthttp://www.newegg.com/Product/Product.aspx?Item=N82E16820231443&cm_re=ripjaw_X-_-20-231-443-_-Product
RAM speed will not effect processor performance, and don't in any circumstance buy more than 8GB (any more is an absolute waste for current programs / games for 99.99% of users)