Right, lighting helps. There are a bunch of factors contributing to the end 'sharpness' result.
-Focus is obviously HUGE
-How steady your camera is
-How much light is on the subject
-What lens you are using
-How much sharpening you add in post
As far as camera bodies go, don't sell your XSi short. Here's a shot just the other day I did with my
XT-- yes, that ancient entry level 8 megapixel dinosaur.
Here's a 100% crop of the eyes:
I haven't added any sharpening in post yet (will do so if I print it)