I currently have a Yamaha LPX-510 with a 106" Stewart Firehawk. In my den I also own a Sony KDS-55A2020 55" Grand Wega Sxrd Rear Projection HDTV. I use to think the picture on the Yamaha was the b*lls. That was until the sony arrived in the house. I know unless I spend big bucks I can't get the same picture but I'd like to get closer than I am now. Any thoughts on what I should look at. I would like to keep the Firhawk in place and spend 5K/7K.

If you like the SXRD, definitely consider the VW60. My VW50 on a 119" HP screen was outstanding (blacker blacks, smoother image, and no SSE) compared to the 60" XBR2 in my house. Also. the RS1/RS2 or Panasonic AE2000u (which I found to be better than the VW50 in many ways: sharper, better color, etc). The Panasonic will be around half the price of the lower end of your budget.

Generally RPTV's will appear to look better? Why? They are smaller so you have more light output, smaller pixesl (thus sharper), etc...

But size for size usually front projection will do better. As Rob said, the Sony Pearl (now Black Pearl) is a good option if you like the look of your current Sony.
They are also less money than you probably paid for your Yamaha back when...