When Apple released the 3rd Gen iPod Touch, they didn't update the 2nd Gen 8GB with a newer processor, or with Voice Commands, it was maintained as a 2nd Generation iPod Touch while being advertised as a 3rd Gen Device even though no changes were made. However, the 32GB was updated, and a 64GB iPod Touch was introduced.
Basically any iPod Touch that is 8GB in size that is in the 2nd Gen form factor (no cameras, tapered edges) is compatible only with 2nd Gen parts.
If you purchase a 3rd Gen part for your 8GB iPod Touch, it will NOT work. The connectors are that of the 2nd Gen and a 3rd Gen part will NOT fit.