30 (or 25) FPS is normal for SD video. The only time you go higher is 50 or 60 FPS for the higher end HDTV systems.
If it's getting jerky then things are going out of sync between the video grabber and the game and it's dropping frames. Drop the game's video quality settings and try and give as much CPU to the grabber as possible. It'll also needs good hard disk speed, preferably it's own disk to record to.
If you really want to keep the frame rate up it's worth getting a separate computer to do the grabbing, and split the video out from the game computer. I use a Canopus ADVC55 to turn either NTSC or PAL video signals into a Firewire video source that can be captured right into a video editing program.