Question about improved video through a receiver or not

I recently replaced my Panasonic 42" 720P television with the new 1080P 55" Panasonic TC-P55VT50. I also purchased the TiVo Premiere XL4 DVR. My Verizon FIOS is connected to the TiVo. I also have a Sony Networked Blu-Ray player. It's all connected to an older Onkyo TX-SR803. The video from both the TiVo and Blu-Ray player is connected to the receiver through HDMI cables. The receiver then outputs the signal to the TV from one HDMI out connection. When setting up the TiVo, it gave me the message that the receiver did not support 1080P 24fps, which the TV does, so it switched it to 1080i. I also run digital optical cables to the receiver from both the TiVo and the Blu-Ray player.

My question is this: Should I replace the Onkyo with a newer model (NR818?) and continue to run both audio and video through the receiver or am I just as well off running the HDMI cables from the sources directly to the tv and keeping the audio running through the optical cables connected to the receiver? Does a receiver make that much of a difference in the video or is it just for switching convenience? I assume the audio will be the same either through the optical cables or the HDMI cables.

Thanks for any help anyone can offer.

Thank you for your reply. That's kind of what I've been thinking is to just keep the HDMI going straight to the tv and run the audio through the receiver. But, one question, you referred to my optical audio connections as lossy. Does that mean the HDMI audio is lossless and that would be an argument, although not major, to run everything through the receiver?

Thanks again for your help.

Thank you. Now the decision is whether or not to spend the additional $1000 to get the new receiver.