A warm welcome to the CameraLabs forums.
I'll admit at the outset that I'm in the "happy snapper" category and so am not qualified to advise which way you should go but maybe I could ask a couple of questions?
My first thought, having had a look at your web-site, is that I like your style.
The Flash pages don't allow pixel-peeping so my first question is what aspects of your picture quality is causing concern.
It's mega-expensive to obtain a significant increase in sensor resolution. If you are considering a medium format camera then you might be interested to read this Mamiya ZD digital back review
by Frank Doorhof (and check out some of his work here
). Then, of course, there is the Canon EOS-1Ds line or the new Nikon D3.
For great IQ without breaking the budget so badly have a look at Gordon's Canon EOS 5D upgrade
feature. Some great prices are to be had on the 5D at the moment with the anticipated launch of it's successor fairly imminent.
I guess the trouble with all of the above is that you end up throwing away your current investment in glass unless you keep the 400D/XTi as a spare. I upgraded from a 400D to a 40D recently and have been delighted with it but I think it's fair to say that while it offers extra bells and whistles the IQ is not significantly different from the 400D though maybe its high ISO performance is a little better.
You have asked your question about lens upgrades here
where I am sure you will get good advice.
Do you think you need to upgrade your lighting system? Again not my area, but it occurs to me that you might get the most bang for your buck there.
Good luck with your choices.