Hold of on the 40D. The Xsi/450D is a great camera. If it feels too small and light for you try with the battery grip this does add weight and size to the camera. Also if the camera feels too small to you theres also the Camera Armor, it does bulk up the grip some.
As for the lens get the lens with the longer range if thats what your after. Truthfully the glass is more important then the body (if the body is able to produce great pics, which the Xsi/450D is able to do), cheap crappy glass will put out the same quality photos be it a 40D or an Xsi/450D.
Move on to the 40D (or even a 1D) later. You can then keep the Xsi/450D as a backup body.
The settings you can do with the Xsi/450D isnt just the aperture (which is the f-stop), you can also set custom white balancing, setting focal points, picture styles (6 pre-programed styles with the ability to set up 3 of your own custom styles), exposure times, ect... basically everything you can do with the 40D. The Xsi/450D is far from a point and shoot camera, have you experimented with the creative zones yet, more specifically the M (Manual) setting. The creative zones give you the freedom to set up your own custom settings, where as the basic zones let the camera do all the work.
Just because the Xsi/450D is smaller and lighter then the 40D doesnt mean its isnt as good. There are a number of us Xsi/450D owners here who love the little camera, and I am sure others will chime in about the battery grip adding to the overall size and weight. I also believe even the 40D may have problems with tipping forward with certain flashes (and lenses), plus the 40D isnt that much heavier in the long run, only about 9 more ounces (Xsi/450 is 16.8oz and the 40D is 26.1oz), and the size isnt that like the 40D is a couple inches for each dimension larger either (40D is 5.7 x 4.2 x 2.9 in. where as the XSi/450D is 5.1 x 3.8 x 2.4 in., only an average of .5 in. per dimension). Plus if you have a bunch of SD cards then you will need to re-invest in CF cards for the 40D, add in the fact that CF card ports are notorious for getting bent pins. CF cards are also larger and heavier then the SD cards, you can care more SD cards in less space then you can CF cards.
These are just my opinions, you can take them or leave them, it doesnt matter to me since its not my camera. I have an Xsi/450D and I love it, matter of fact I am looking at getting a XS/1000D (with the battery grip) for use as a backup body.