One thing i'll say is this your GPU Will have nothing to do with the Capture of your video... It's all going to come down to the other hardware, like CPU, Ram, HDD speed, and Capture device.. 
and since you have a capture card some of the work load is dumped to that but still need to have the other items up to spec.
 
I will be honest it's been a long time since i've captured Analog video (not since high school)  but my best suggestion would be  this. 
 
If you have a Digital or Mini DV camcorder that has Video in ability. I would dump it to that first. 
 
Cause then at that point it's already Digital and just needs to get transferd from point A to B, Mini DV was nice cause you could easly set in and outs via the time code that was on the tape if you where running Via Fire wire or USB to the capture software. But with S-video it's all Real Time Capture. AKA you tell it start then you hit play on your source. 
 
And if what i'm seeing on your capture board..  all it's doing is providing a video input. it doesn't handle really any of the capture and encoding of the video coming in. Which would be handled by  the CPU and Ram and how well the software that is being used to capture is optimized.
 
if your new set up doesn't work as well. then my main two suggestions would be  these
 
Better Capture software
Better Capture device or Capture card that will handle a bit of the processing (if there are really any that are out there that do that not fully sure)
More ram...
 
Those would be my best suggestions.
 
as for the GPU.. as far as i under stand. that won't even be used at all unless the video editing software you use to Render out the Video is Cuda enabled software. then it would use some of the GPU power to encode and render out the final video.
 
hope that helps a bit  as for every one else. feel free to correct me or back me up or make fun of me if you wish.
 

Just a note on my video capture (Analog to Digital)  experience w/ the 1GB/PCI-E 210 series card.   The video card did  indeed  improve performance of video capture over my first attempt using an AGP 256mb  ATI Rage  GPU  on a similar computer (2.8/HT  2GB ram).   I successfully converted some 8 hours of videos.  I have to do it again as I incorrectly connected the audio line but the video came out great considering the tapes were 20 years old.  The new computer is also a Dell 2.8/HT but only has 1GB ram but a PCI-E port and the GeForce card).
Despite NovaKit's comments on the GPU card having no bearing on Video Capture, I disagree.    I'm not a techno-whiz but  everyone else I  read on many video forums claim that  that the GPU ram and  PCI-E port both heavily affect the video analog to digital capture process.
nh

NovaKitFox
as for the GPU.. as far as i under stand. that won't even be used at all unless the video editing software you use to Render out the Video is Cuda enabled software. then it would use some of the GPU power to encode and render out the final video.

 
[OS] 7or8 64bit
[CPU] 2or4core 3GHz+
[RAM] 2x 4G+
[VGA] Cuda
・Vegas Movie Studio Platinum 12　(Only to Sony AVC.
・Badaboom
・Super LoiLoScope
・PowerDirector 7+ 
・ArcSoft TotalMedia
・Theater vReveal
 
--------------------------------------------------------- 


Geforce 210  
DVI, VGA, Display-Port ( HDMI 
InternalAudio Input for HDMI 
Single-slot Width 
105C Maximum GPU Temperature  
300W Minimum System Power Requirement 
  
Geforce 210  
[GPU Core] GT218 ( CUDA Cores 16  
[Transistors] 727 million 
[Process] 40 nm 
[Vertex Pipelines] 16 
[Pipeline Layout Scalar] MADD+MUL 
[Fragment Pipelines] 16 
[Texture Units] 8 
[Raster Operators] 4 
[Pixel Fill Rate] 2356 MPixels/sec 
[Texture Fill Rate] 4712 MTexels/sec 
[Core Clock] 589 MHz 
[Shader Clock] 1402 MHz 
[Memory] 512,1024 MB 
[Memory Clock] 500 MHz (1000 DDR) 
[Memory Bandwidth] 8 GB/sec 
[Memory Type] DDR2 
[Memory Bus] 64 bit 
[FLOPS] 44.864 GFLOPS 
[DirectX] 10.1 
[OpenGL] 3.2 
[PS/VS Version] 4.1/4.1 
[Interface] PCI-E 2.0 x16 
[Max Power Draw] 30.5 W 
[Noise Level] Silent 

・9400GT≦ GPU Core GT218 = Geforce 210 
・Geforce 210 - OpenGL 3.1 / DirectX10.1  Windows 7 
・9400GT - OpenGL 2.1 / DirectX10  Windows Vista