Would upgrading to SSD solve my performance issues? My MacBook Pro specifications are:
Core2Duo 2.65 Ghz
4 GB RAM
320 GB HD  

Performance is killing me literally. Really long response time.  

Do you suggest to replace the Hard disk with SSD one ? Is it going to solve my MBP performance issues?  



What is getting slow

I'm a software developer (code compiling, building, Xcode, IntelliJ, Sublime Text 2, tens of Chrome tabs, iTunes, Preview).  

It could takes seconds to switch between running applications. A lot of spinning colored-balls.

It takes seconds to launch an application.
A RAM upgrade is inexpensive (~$70) compared to an SSD (~$1 per GB e.g. $250 for a 240GB) and based on your pagefile usage and available RAM I would start there.  That being said, an SSD is a massive performance upgrade and worth every dollar in my opinion.  I won't use a machine without an SSD anymore.

Links to: RAM and SSD.

Could you provide some more details as to what specifically is slow?