It is possible for SSD's to function on SATA 1 but it would pay to check this out with the specific manufacturer of the SSD you are considering. All Corsair SSD's are compatible with SATA 1 according to Corsair. http://www.corsair.com/support/faq/solid-state-drives/
You will pick up a substanial performance increase over a HDD especially the 5400 RPM drives that come standard with laptops. If you were doing large file transfers your SATA 1 may be limiting but even so you will still get a good boost. If you consider the fact that file transfers would be limited by other factors such as your NIC or USB 2.0 for external drives your real world performance limitations would be small. The best part of the SSD technology its vastly superior seek times. SATA 1 will not limit the seek time advantage.
Keep your original drive so if you decide to sell the laptop later you can whip out the SSD and throw it into your next machine.