I replaced the HDD on my 2008 MBP with a Samsung 840 Pro 240g SDD. I also maxed out the ram and put on a fresh install of Mavericks. Yes, that computer is only SATA I and cannot take advantage of the drive speeds, but let me just say it is, with that mod, like a new computer and I plan on getting many more years of us out of it. The added bonus is you can then use your old HDD as your time machine drive.

The major noticeable difference is in the drive access speed. Startup from cold is under 20 seconds. Programs open almost instantly. Even the processor hungry ones. I can't recommend making the switch enough.

N.B. - it's easy enough to do yourself and I comfortably did it by watching a youtube video once or twice., but be careful with the ribbon cable. I broke the connector at the motherboard coming out. It was a cheap fix, but put my box out of commission for a week while it was shipped.