I have so many friends that have the 6600, I should go and get one! With that said, I would KEEP your other machine and buy the 6600. (or buy a vintage machine and keep 99% of your money)

Keep your singer for the freearm and various stitches. Have you considered a high speed straight stitch machine? I bought a Juki TL 2010Q three months ago and it is amazing...$900. I have yet to learn to FMQ. It has most of the features you want. Two other machines comparable to this are the Janome 1600p and the Brother 1500s. Just something else you might want to consider for a lot less cost. I myself did not want computerized or tons of stitches. I didi most of my research on this board. I love my Juki.

I definitely think you should follow the suggestion to look further. You will be spending significant money. I made the mistake of getting in a hurry and purchased what I thought was ok and hated the thing. I'm still stuck with it because I wasn't able to sell it. And the brand I ultimately chose couldn't take this machine on trade. I wasted $2,500 on a machine I only get out when I need to mend jeans. Try lots of brands before you choose and take the time to think about it. Sticking with the brand you've had may not be the best plan or it might be. But you won't know til you try them all.

Have you considered a Pfaff Quilt Expression 4.0? You pretty much described it with your "dream machine"

So many choices, good luck! I love my vikings!!

I did consider the pfaff QE4. I saw some scary reviews about this model when it first came out (mostly about shredding thread and a defective take up). The main thing I didn't like about this model is that the bobbin cover is not flush with the front of the machine and I can see how the "bump" would flip seams the wrong way. I tried it out at a local dealer and it was really annoying. The 7700's bed and needle plate fit seamlessly together.

Hmmm...not sure what you are refering to. I haven't noticed a "bump" on mine and have not had a issue with the other problems you mentioned.

I bought a 6600 in the fall of the year and I have been very pleased with the machine. It has done everything that I needed it to do. The AccuFeed system is awesome.

I bought the 6600 only to trade it in for the 7700 two weeks later. The reason was the harp space and the price was not that different. (500) I like the fact that the 7700 has more quilting stitches, and satin stitches. I also like the lighting better. I have a Bernina so the free arm was not an issue. I tested buttonholes and the 7700 were better.
The things I like about the 6600 I had more then the Horizon are, one it was slightly quieter. I liked the vertical thread spool holder much more. I liked the bed more, as fabric slides around easier when FM Sewing. I like the push buttons, It is more intitutive to me, and feels more direct to get where you are going faster.
Other then these comparisons the machines are very similar. I do like them both very much. If I were on a budget I would have no hesitation in buying a 6600 for half the 7700.
I do think you can get the 7700 for a lot less (more like 2600) Floor models go for even less.

Well, my decision just got a lot easier. I found a Janome dealer in my area who will sell the 7700 for $2,000 without any classes or warranty (in addition to the Janome warranty), $2,200 with a 2 year warranty, or $2,300 with the 2 year warranty and 3 classes. I'll definitely be going for the 7700, but now I just need to decide if it's worth a couple hundred extra dollars for warranty/classes.