the 750 overclocks well, and 4gb ram is more than enough unless you are doing something like VM's..... What you should upgrade is the GPU, unless your monitor is low res... in that case upgrade the monitor and the GPU... but that's only if you are gaming... if you aren't gaming, but using it for other tasks where the monitor/graphics are not as important, and you still want an upgrade..... go with the i7-875k. There are some decent deals for it on ebay from people who bought it at microcenter...
If you aren't gaming and you aren't doing other cpu-intensive tasks, I would recommend NOT upgrading.....
P.S. What are the ram specs, mobo specs, etc. [More information]