You can use a thing called a Static Route to indicate which connection you wish to be used (if they are both on the same subnet), or you can manually modify your HOSTS file (if they're on a different subnet).
For starters, it's generally a good idea to have the crossover connection (FYI Gigabit does not require a crossover cable, it will automatically crossover itself) on its own subnet. E.g. if your normal network is 192.168.1.x then use 192.168.2.x - this way there can be no confusion for the gateway.
If you have the two connections on different subnets, then the best way to guarantee the gigabit connection to be used is to add an entry to your HOSTS file.
Browse to the following location on your computer:
In there there is a file called 'hosts'. Change this file so that it's not read-only and open her up in notepad. Then add a line that looks like the following:
Except replace the x.x.x.x IP address and the SQLServerto be the IP/hostname of your server. Because that subnet exists exclusively on your crossover network, Windows will be smart enough to know to sent it over that nic.
If for whatever reason you can't seperate the subnets (i.e. they are both 192.168.1.x) then you can use a Static Route to achieve this (You can do this as well if you want, but it's fairly redundant). To add this default route, open up your command prompt (as administrator if this is Vista with UAC) and run the following:
route add x.x.x.x mask 255.255.255.255 y.y.y.y -p
Where x = IP address of the SQL server's gigabit connection and where y = IP address of your computer's gigabit connection
This will tell Windows that when you request the IP address of the SQL server, to run it over the gigabit network.