I dont believe the mining option in the Qt wallet will do GPU mining... I think its only CPU.  The better way is to set up QT as a server and then attach cgminer or similar to it.  If you are running multiple machines then you will need some private pooling scripts running like http://poolserverj.org.

You can find instructions on setting up the QT wallet for solo mining here https://bitcointalk.org/index.php?topic=83371.0

Pools are still a better bet.  You may loose a little in fees but you can see some payoff for your work almost instantly.   http://www.litecoinpool.org is by far the best pool but I believe registration is still locked.  You can join chan #litecoinpool on http://webchat.freenode.net/?channels=#litecoinpool and ask pooler if he will let you in.