Upload & Sell: On
anthony -- Let me see if I can summarize what you want:
1) a camera no bulkier than the 40D, and preferrably less bulky
2) full frame or APS-H sensor
3) not to exceed one thousand pounds UK (about $1600?)
4) camera to be used out and about, including some buildings and detailed subjects
You currently have 40D and G10 as your main cameras.
The 1Ds2 and 1D3 are terrific cameras, and they are in your price range if you are patient in looking around. Either will provide far superior imaging to your 40D, and have completely pro features. Neither will be small or light, nothing like your 40D with its crop sensor.
The other option is as Ernie mentioned, getting a Nex or M4/3 camera with some nice glass. Not cheap when you consider starting a new system and not as good of IQ as the 1Ds2 or 1D3. Still, it would be small in size overall. Better IQ than your G10, but at a significantly higher price. Not nearly as pocketable as your G10.
Another thought is to get a used 5D original and use it as your minor upgrade. If you aren't very familiar with full frame capabilities, this camera will open your eyes at a budget price. And about the size of your 40D. This might be your best option.
And finally, the "do nothing" approach to IQ -- buy another 40D or one slightly older like a 30D (even a 20D), to have a back-up. But it sounds like you really want to up your game by moving toward larger sensors.
As Dan said, if you move up in IQ and features, your 40D will undoubtedly be used less and become the back-up. It doesn't really make sense to keep a better machine on the shelf gathering dust while the less capable does the work. Does it?
Just so you know, I'm in a similar position to you. I have a G10 and love it, using it exactly like you do. Then I have my better IQ cameras, and those are older 1D2 and 1Ds2 models, along with "back-up" 60D and 5D. So, I certainly can understand your inclination and types of shooting you do.
Hope everyone's ideas are helping you to decide.