If you are still using hard disk drives, then just about any ssd will be an improvement.
A solid state drive that is SATA III (6 Gb/s) capable is backwards compatible with SATA II (3 Gb/s) ports on your motherboard. However, there will be a performance hit until you upgrade to a new motherboard.
Right now the Crucial RealSSD C300 128GB SATA III ssd is a pretty good value. From time to time newegg.com will have it on sale for $199.99 and free shipping. You just have to be patient and check every day to get the sale price.