Several points here.
1) Teleconverters multiply the focal length of the main lens but always degrade the image to varying degrees. The more the multiplication the more degradation, do not be tempted by anything stronger than a 1.4 converter for this reason or you will be disappointed.
2) They work best with prime lenses but a 1.4 should be OK with your zoom but there will be some loss of quality.
3) You always lose light with a converter, with a 1.4 converter you will lose 1 stop of light so your zoom will change from a f5.6 to a f8 lens.
4) With your 350D camera this means you will lose autofocus.
This is because any Canon DSLR apart from the professional 1 series will only focus with a lens of f5.6 or faster..
5) Some converters are 'none reporting' which means they do not inform the camera about the change of aperture but even so autofocus would be slow and erratic. Some people mask some of the pins on the converter to make them 'none reporting' but with mixed success.
6) The focus screen will be darker making manual focus quite difficult. Shooting with a long lens has a steep learning curve due to many factors including very narrow depth of field requiring very accurate focus and a much increased likelihood of camera shake leading to blurred shots. The effective f8 does not help here requiring that you use high ISO settings.
As far as makes go it is best to go for a good converter, I would personally recommend the Kenko 300 pro if it is in your price range. Do not let me put you off, I use one myself and get good results but just be aware they are not a magic bullet for cheap long lenses for the above reasons.
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