You have a 46" 1080p High Def LCD display. Awesome. The only way to improve the picture quality is to be able to look at some truly great HD programming like INHD, ESPN HD, Discovery HD. In other words, instead of buying something to improve the quality, subscribe to the ComCast digital plus package and budget from there.
Make sure you get the high def STB (Set Top Box). Then, use the digital connection (HDMI, or, DVI to HDMI cable depending on the box) to connect to your Sony. Run the STB audio output to the TV. You will get audio.
You can get the HDMI and audio cables at www.monoprice.com
. Use the additional money you save by not getting Monster Cable and apply it towards the cable bill.