Hey and if you are looking to do that but also want to ugrade your mobo you should just get a new mobo with a built in gigabit networkcard, somehow mine screwed up and i had to go back to a pci 10/100 card which sucks cus i can tell the difference when i play games. ANd the onboard was 4X faster which pisses me off, i need to fix that.
I'll assume you had a gigabit switch etc., to achieve a 'true' 1Gbps rate.
If your nic was set up to run @ 1Gbps, and you notice a difference with a
10/100, check to see if your nic supports jumbo frames. Jumbo frames are designed to move huge amounts of data / packet - if your nic was using that for gaming, you will notice a slight decrease.