The G7 is a great camera, one of the best of the pack when you talk about compacts.
Great looks, size, features, etc. Probably about the only thing being complained about by everyone... and unfortunately, its a pretty big omission, is the lack of Raw format.
Now comparing the G7 with SLR, I think thats quite a difference.
The main one being sensor size. The minuscule sensor of the G7 whether its 10 mpix or 20 mpix is simply not gonna beat the APS-C of an SLR, even a D40 with 6Mpix would easily give out as much detail as the G7. Maybe in a super bright outdoor you can still get a good performance out of a compact, but indoors, or portraits you are not gonna beat the sensor size difference.
Dont just see the megapixel count, sensor size is very important.
Back to your camera needs...
You mentioned an SLR is expensive... well it doesnt have to be, its only expensive if you start buying the top end accessories, prime lenses, etc. But for the most part, you can live pretty well with the kit lens.
And you mentioned you take portraits, wildlife, landscapes, thats like all the major differing types of photography already, exactly why the SLR is invented. So you can change the lenses to your needs. The lens you need to shoot a landscape and a wildlife is vastly different.
As for size... if you are singling out Canon then have a go with the EOS400D.
If you are willing to look at other brands...
The Olympus E-400, or E-410 (newer one) are the smallest SLRs on the market.
Try it you'll be surprised how small it is.
Also try Nikon D40, or D40x, volume wise its comparable to the Olympus, while the Olympus is thin and wide, the D40 is a little shorter in width but quite a bit thicker.
I think those 2 especially the E-400 would fit your bill as an "almost compact" SLR