OCZ classifies the Agility 3 as a mainstream ssd. It is an older model. It is being replaced by the Agility 4.
The user feedback at newegg should be taken lightly. They typically leave out vital information. You should be reading technical reviews. I maintain the ssd database listed in the sticky at the very top of this form section. Here is the link:http://www.johnnylucky.org/data-storage/ssd-database.html
Scroll down to the OCZ section where you will find links to technical reviews.
You are right about your motherboard being an oldie. However, it does support SATA 2 3Gb/s. Modern SATA 3 6Gb/s ssd's are backward compatible so you will be able to use it. However, there will be a slight performance hit. For doing ordinary stuff you probably won't be able to see the difference.