Canon currently make EF and EF-S lenses, both of which fit their amateur EOS cameras (up to and including the 7D).
(1000D, 1100D, 300,350,400,450,500,550,600D, 20D, 30D, 40D, 50D, 60D, 7D all work with both because they have smaller sensors)
Anything beyond the 7D (5D series, 1D series, 1Ds series) will not accept EF-S lenses. This is because they have bigger sensors and the lenses do not physically have a wide enough view.
The Canon FD mount is different to the EF/EF-S style mounting and you need to find an adapter to fit these.
It may not be worth the hassle and the new lenses obviously do have certain advantages over old lenses (controls through camera & image stabilisation).. it kind of depends what you want out of your camera and what FD lenses you already have. Bear in mind that if you buy a new camera with a smaller sensor than the size of your film frame, the view will appear to be zoomed in. Therefore an FD 35mm becomes roughly a 50mm on a 600D.
Hope that helped