If you're just watching TV, a gig of RAM is plenty. The 2400+ CPU should probably be fine as well.
For hard drives, ATA-100 and ATA-66 are the interface (or burst) speeds, which tell you virtually nothing useful except for the approximate age of the drive. The real variable is Sustained Transfer Rate. Having said that, once again you are probably fine with that too. You should definitely get a tuner card with a hardware encoder to handle the analog signal. (Since the card does the work, the CPU is not an issue.) It will probably be in the 6Mbit/s range. OTA HDTV is (I forget) 20ish Mbit/s, straight out of the air and onto the drive. So that's about 3Mbyte/s. A slow ATA-66 class drive might be 10Mbyte/s, so that's plenty. Compare those bitrates with what you successfully captured before.
The problem is the video card, especially with 1080i HDTV. nVidia brands its video processing as PureVideo; looking at the
support matrix the features you want for High-Definition Content are MPEG-2 Decode Acceleration, Spatial-Temporal De-Interlacing, and ideally Inverse Telecine. So that's a minimum 7600 GT or 6800 Ultra.
BeyondTV will automatically "ShowSqueeze" recordings into DivX (not sure if it's the exact flavor you want, and that's when CPU power will matter). I don't recall any support for downloading shows, but if you place the video files in the right directory, you can watch them through the interface.