Most people install the gui client at first since it is the easiest to setup.
Main problem with it is the eventually it will have problems if you run any other apps that use open g/l since m$ support for it is not very good.
Also you cannot run 2 instances of it to take advantage of your dual core.
To switch to the smp client, let it finish its first wu (which can take several days) and make sure it doesn't download another - i think there is a setting in the gui for this I just can't remember where it is.
The smp client will produce at least 2-3 times more than running 2 standard clients, or 4-6 times as much as 1 gui client. Your rig could produce ~ 1 smp wu per day if running 724 and smp wu's are worth from 1100-1760 points. 1 gui client will get ~ 250-400 points per day on your rig depending on the wu. I am not up to date on what the range is for current single core wu's since I run only smp except for a laptop I have.
running the smp client requires you to have a logon id and password on windows.