Your EF lenses should work okay - some older lenses you can only use with manual focus & some sigma lenses need to be rechipped in order to work with digital. The man in the camera shop would know if you give him the lens model number
"Digital noise" refers to the nasty coloured speckles you sometimes see on digital images. Like if you look at an area of blue sky at %100 view you'll see blotches of other colours, instead of all smooth blue shades, which will make the pic look a bit dirty, like a bad case of film grain. It's far less of a problem on DSLRs because of the larger sens