The SATA controller in this system is old enough that it's likely 150 MBps - not 3G.  There's no way it's 6G.

You should have a sata 2 motherboard interface.

That being said, you can use either a sata 2 or sata 3 ssd but only expect sata 2 speed. On my Inspiron 1520 I get 250MB/s read and 150ishMB/s write.