Yes, you can swap out your current HD for a SSD. Not sure if you'll gain that much given the fact you'll loose space.
A better option maybe getting a hybrid drive which has an SSD working in conjunction with a platter drive in one unit. Seagate, Western Digital, Toshiba & others now make nice units. Early versions had a few bumps.
The other option here is to swap-out your optical drive for a 2nd drive carrier and put the SSD the SSD there leaving the HD where it is.
Just be careful on the SATA I/O speed of what ever you put in. Most current drives (HD & SSD) are SATA III (6Gb/s) and depending on your system you have either a SATA I (1.5Gb/s) or SATA II (3Gb/s). You need to lower the drives SATA I/O speed to match your systems SATA I/O speed.
I gave up putting SSD's in the older MBP's (pre-unibody) and instead I put in hybrid drives (500GB or 1TB). Between the fact the older drive was likely to fail soon due to age and most of my users didn't want to give up on the optical drive so they could watch movies on the long plane rides.