how fast is a 1Gbps lan?

I have Comcastmodem by scientific alanta to Linksys router WRTS4G --- lan1 to SonicWall TZ170

100Mbps to 1Gbps Netgear switch to -------------------------lan2---- PC with 120GB IDE ST3120020A disk Linux box w. raid5 drives

The netgear box shows both the linux box and the PC connected at 1G (I am assuming full duplex)

with Cat5e wiring.

BUT the speeds between the Linux box and PC never exceed 40Mbps (5MBps).

Is this due to the disk drives or could there be some other issue?