silly question(I'm a noob):I don't how to overclock
and how much does a sandy bridge cpu cost?
That depends on your system:
Motherbord:H67 based would cost around Rs5000-6000k
P67/Z68 based starting from Rs10k and above.
CPU:if you chose H67 bsed chipset,then you should go for i5 2400(NO OVERCLOCKING),price around RS 9k
if you choose P67/Z68 ,go for i5 2500K/i7 2600k.(EXTREME OCing),Price around Rs10K.
Why wouldn't overclocking help if he is crossfires?
@OP: Sandy Bridge is definitely the way to go if you can afford it. I would recommend the i5-2500k. It is very easy to overclock and with decent cooling should reach 4.5ghz without much of a problem. Here is a comparison with the 955; http://www.anandtech.com/bench/Product/288?vs=88
The above ignores overclocking so the difference is actually significantly greater if you were to do so.