If you specifically want to shoot Macro, it's possible to get a dedicated Macro lens for you XSi for a non-crazy amount of money.
e.g. Canon 50mm f2.5 Macro ($260) or Sigma 50mm f2.8 Macro ($300)
Or a set of extension tubes from $80-$160 depending, which can be used with your current lenses.
However to be perfectly honest it's a very common theme on these forums to hear that someone moving from a P&S camera to an SLR finds their pictures get a lot worse before they get better. 99.9% of the time this is not because there is anything wrong with their camera or lenses, but simply because they haven't learned to use their equipment yet.
A DSLR has the potential for far better images than a P&S, but without the required skill behind the camera the pictures will be worse than those from a P&S.
The most common response (particularly among those who frequent forums) is to believe that there is a problem with the camera or (more likely) lenses; that they have received a bad copy.
The XSi and lenses you have are more than capable of producing fantastic images. Check out pBase or Flickr for abundant proof.
Or perhaps you are part of the 0.01% who really did get a bad lens or camera.
If you post some samples of really great images which were let down by the limitations of your lenses or camera then perhaps we can help.
Last edited by peripatetic; Nov 30, 2009 at 2:14 AM.