The 2500k has an unlocked multiplier so it is extremely overclockable,but since the 2500 has a locked multiplier,it is only slightly over clockable,since the most stable BCLK I have seen was 105 compared to the stock 100 so at max you can have a 5% overclock.
Before upgrading I would like to know which PSU you have,I know it has been able to run the power hungry HD4870,so it would be more than enough for the modern GPUs but still if it is not of good quality then maybe you should think about replacing that as well.
Some good brands PSU brands would be Seasonic,PC Power and cooling,XFX,Silverstone,Antec and Corsair.
And a 550W would be more than enough and future proof.
But if your current PC is not causing you any problem nor lagging anywhere then why upgrade??
If there is no problem with a thing then don't fix it.
Anyways since the Zambezi would be releasing soon and Intel's new 2011 in the Q4 I guess ,
with your config it would be better to wait,and save up.
But I would strongly recommend waiting since as of now also you PC should not have any problems playing really demanding games at medium-high settings.