Just as Knytestorme said...

Open up your Task Manager and look at your CPU usage while playing. If you're pinned at 100% then a video
card, most likely, isn't going to magically take load off of your CPU. Upgrading a video card is usually done so
people can play at higher video settings which, in turn, raises the system load overall.

Remember, with RIFT (by default) your background game FPS is severely lower than your foreground FPS; so,
if you're going to be looking at your Task Manager and the game window isn't in focus you might be getting
wrong readings.