I wouldn’t knock your kit lenses too much. You will see lots of reviews describing how, on a test bench, under lab conditions, they don’t match up to the best. But n real world conditions they are capable lenses and the best part is that on the 450D cropped sensor you are using the sweetest (central area) part of the lens.
Re macro …… if you are looking to combine a zoom with a macro lens then you are unlikely to get true macro. True macro is generally accepted as being 1:1 sensor to true-life ratio and beyond (i.e. an insect which is 10mm long will take up 10mm on you sensor). Some specialist lenses give up to 5:1 magnification but you are talking mega expensive with those. The term macro is banded about fairly loosely by lens manufacturers for zoom lenses when what they actually mean is ‘close focusing’. An example would be the Sigma 17-70mm f2.8-4.5 ‘macro’ where the magnification ratio is actually 1:2.3 so that every mm on the sensor is capturing 2.3mm of the subject
Sig 17-70mm price
Sig 17-70mm write up at DP Review
For a good true macro lens you could do worse than consider these three:
Canon 100mm f2.8 macro
Tamron 90mm f2.8 macro
Sigma 105mm f2.8 macro
If / when you get around to seriously considering a macro lens then stick a few questions in the equipment section and you’ll get loads of help
Cheers, Dave B.