See if you can find a port analyser/ monitor off the net and look at the latency. If it dips when you see the juddering then you know it is the install and not the hardware. i would blame windows every time regardless if the hardware was naff :-)

If it is the install then i should imagine something is taking priority over the card causing the latency issues. Investigating the software cost nothing before you spend any more on hardware. See if there is a newer driver for the card?