Feeding HDTV and SD Tv

I have an HTPC and am using onboard video (Radeon 1250) to feed multiple tube tvs using S-video output on the MB. I got a new 50" plasma. The onboard supports HDMI as well as S-video. However, when I try to use both the S-video and HDMI

1) It seems a little shaky in terms of the signal, sometimes cutting in and out (maybe a power issue?)
2) The HDMI signal gets downgraded to 800x600. (I assume because s-video cannot handle higher).

My question is how do I get this to work and duplicate the display and all tvs?

I have read conflicting reports that a second video card in the same line (radeon) using hte same driver will allow me to dupicate the display and use the onboard for S-video and graphic card for hdmi (or vice versa). But how do I use the same driver when they will be different chips (1250 vs probably a 3450)?

Any other options other than expensive video converters?