Sure, you have a Gigabit card, but how can you check to see what speed you are running at in Linux? All your networking equipment must be Gigabit for it to work. There are two tools to do this, /usr/sbin/ethtool and /sbin/mii-tool

sudo /usr/sbin/ethtool eth0
Password:
Settings for eth0:
Supported ports: [ MII ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Half 1000baseT/Full
Supports auto-negotiation: Yes
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Half 1000baseT/Full
Advertised auto-negotiation: Yes
Speed: 1000Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD: 1
Transceiver: internal
Auto-negotiation: on
Supports Wake-on: g
Wake-on: d
Current message level: 0×000000ff (255)
Link detected: yes

Enjoy.