What SSD should I get for replacement of old cylinder HDD (sata 2) for Intel Centrino laptop in terms of value for money? Should I bother getting something like Corsair Forge series 3 with read speeds of up to 555Mb/s? Will this laptop (Sony VAIO VGN-FW23G) will ever be able to read that fast? What's the max speed would be?
Also, will I have any trouble replacing the hard drive? Any predictable issues?

Thank you!

The series 3 is a Sata-3 drive which they say reads at 550MBps (megabytes / second), not 555Mbps (megabits / second) which your controller will not do. If the controller is a Sata-2 controller, it will handle something around 350MBps.

The other issue is whether or not this drive will work in your Sony. A Sata-3 device will work with a Sata-1 or Sata-2 controller, but only at the controller's speed. However, Sony Vaio systems have been notorious in the past in not allowing components that are not Sony provided or authorized. My wife had a Vaio some years ago and this was a real problem for her. So, I think you need to contact Sony about whether or not a non-Sony SSD will work in this system.