Speed of the second card should have no impact on performance, because it doesn't do any processing of the image.
How much processing the secondary card does depends greatly upon how you set up SoftTH.
If you're rendering 5040x1050 and you're sending over full 1680x1050 frames in RGB32 mode to the secondary card, then PCIe bandwidth and the cards own memory bandwidth are all that's important.
If you're rendering 5040x1050 and you're sending over 1440x900 frames in RGB16D mode (settings that save on PCIe bandwidth), then the secondary card has to do up-scaling and dithering.
As I noted, going from an 8400GS to a 9800GT as my secondary card gained me about 5 FPS. More PCIe bandwidth, more memory bandwidth, faster GPU. It makes a difference.
Edit: I'm not recommending a 9800GT as a secondary card for SofTH, but something newer/faster than an 8400GS might be beneficial.