When the ReplayTV records to MPEG on the drive it compresses the video. Then going from the TV to the video card over RCA will reduce quality again, and the video card will then recapture and recompress the video again. This is called generational loss.
Compression causes a loss in quality.
RCA cables cause a loss in quality.
The loss in quality from your ReplayTV to the TV screen isn't noticed because the TV has very poor resolution, and is very forgiving of on-screen artifacts. But it does add up. The more generational loss, the poorer the image even on a TV.
The only way to avoid the generational loss is to make the first generation MPEG be the one on your computer hard drive, and the only way to do that is to take the ReplayTV out of the equation. One easy way to do this would be to upgrade one of your systems with generic video to an ATI Radeon All-in-Wonder. This card has the same features as ReplayTV but for your computer. Just plug the cable TV into the video card and set the record time and quality.
If you really, really want to keep the ReplayTV, you could get a card like the Radeon 64MB VIVO (Video In, Video Out), or possibly one of the ASUS GeForce2/3 cards with video in. These cards would do exactly what you had described. They don't have tuners so you have to plug RCA into them. Then they can record to MPEG whatever is coming in. I know the ATI card can even be set to record at a certain time. It can't set the channel, but if you have a spare VCR that is connected you could pre-set the channel on that. The only loss with this setup will be from the RCA cables. However, with a ReplayTV the video will suffer from generational loss.
So what are you planning on doing with the MPEG files once you get them on the computer? Make a VCD perhaps? Those files get pretty big to just keep around.