Buy a $20 audio card with SPDIF out or look CLOSELY on your mobo and see if it has a SPDIF output header (2 pins although the connector often has 2 or 4 pins) - if you have the header you can fab a cable to it for coaxial SPDIF out and connect to your O's coaxial (orange RCA) digital input for DD & DTS 5.1 audio.
The 3800 is weak and recording to a HDD is not the bottleneck (recording is a snap and uses nearly zero CPU)... it's video decoding and you may be ok w/the 210... 1080i will be the harder as you have to deinterlace the video to watch it. Personally I don't use SAF, instead I like the LAV for its splitter and use the MPC-HC filter as my video decoder, which also does a fine job of any .264 material you may acquire. As far as just watching OTA ATSC TV I'd start with the LAV (full audio, video & splitter) and see how that goes!
As a test I'd get MP installed, channels scanned and use the TV-Server to PREVIEW a local 1080i channel. Then, in the main MP app record a 2 minute clip from a 1080i channel. This will act as a constant for you to assess different codecs and I'd open TaskManager and play this clip in the desktop version of MPC-HC - watch its CPU usage and this should be a good indication of how well your e-machine will perform!