The 40D is 3/4 of a decade old, and back from the 10-megapixel era. Nikon has moved to 24-megapixel sensors made by Sony, or Toshiba, in almost all of their small-body cameras; the Df and D4 currently use 16-MP full-frame sensors. You're riding a pretty old horse, but a new saddle might feel nice. But still...the 40D is now pretty far behind the technology curve as far as really being able to "leverage" even f/2.8 pro-grade zooms. The upper ISO limit on the 40D might very well be considered ISO 800, as long as the light is "decent". The problem for Canon shooters is that Canon has re-used basically the same sensor in every small camera since 2009, and then added 2 million pixels to it for on-sensor focus detection that really helps VIDEO autofocusing more so than image quality or ISO capability. The 70D with its touch-screen AF in video mode is kind of a neat feature-rich Canon.
I know where you are coming from. Let's just say this: the 18-55 is an $89 lens in Nikon mount this month; the 55-200 f/4.5~5.6 Nikkor is at $99.96 right now. Nikon does NOT MAKE a cheap, plastic 50mm like Canon does, so there is no el-cheapo Nikon 50, only a semi-professional grade $169 50mm f/1.8 this month, right now. So...you are in effect, holding your entire photography hobby "hostage" to a trio of the cheapest Canon-branded lenses ever made.
Right now, as in today, a D3200 + 18-55 + 55-200 Nikon kit from B&H is $499, shipped for free. The newer D3300 body-only is $499, and the D3300 has the best viewfinder and fastest shooting of the entry level models Nikon has ever made. In effect, you could go back to a Nikon D3100 for $399, and be ahead of the 40D, image quality wise.
I dunno...it's unpopular to bash Canon for its lack of sensor development. Body-wise the 40D was a nice, mid-level $1,599 camera when it came out, but you have reallllly cheap lenses on it right now. I think a Sigma 17-50 f/2.8 would be a nice lens to use on a 40D, as would be some FAST primes like the Canon EF 35mm f/2, and the Canon 85/1.8. The 60D and 70D offer about the same sensor performance as one another, and are nice cameras. USED 7D cameras are flooding the used market right NOW, bringing about $450-$550 today; my local store has 4 of them for sale used. Again--you are currently using three of the lowest-spec'd, lowest-cost lenses; that is what you've hitched your wagon to.