As others have stated they are both of equal ability as in the respected manufacturers starter dslr's.
Best bet to stick with what you have as more cost changing lenses from what you have and you would need to learn a new menu system also.
If you have outgrown the D40 then save for another nikon upgrade.
If you are still finding it can do more than you need then perhaps some more lenses that are better than the kit although the kit ones are good if used to there ability's.
You make a commitment to a system to some extent when you decide what camera to start with although some do shift camp but there is always the extra expense of doing so unless you drop on a good price composing of kit that is a direct replacement for your existing setup.
I would love a D300 but budget and my lack of ability means sticking with the D50 i have, the predecessor to your d40 but i went for the D50 as it has a motor in the body so could get the older lenses that do not have the motors unlike the DX lenses that do have motors in the lens, although i dont have any of the old lenses the option is there.
The thing with the Nikon is there lens mounting system has been going for ever and some of the old film lenses fit the new digital cameras although as said yours does not have internal motor to the body so would be manual focus anyway even with auto focus for the old film versions.
Sorry for the waffle but gives some background to the nikon sytem i think,
all the best and enjoy.
