Well lets look at it from the logical point of view. You have two consumer grade pieces of glass with an entry level camera. You have two options. New glass or new body.
Lets start with new glass. The xti has a decent sensor in it. It is an entry level camera due to construction and features or lack there of. If you add quality glass the technical quality of the images produced will be limited only by the body which already has a decent sensor in it. Where as the current consumer lenses are not capable of producing the technical quality of the lens you listed as being interested in.
New body. With a 5D classic you will be down to one lens. The 18-55 is I believe an EF-s lens and will not fit on a full frame body which the 5D is. The one lens will be a consumer piece of glass on a higher quality body. The body will be limited by that glass to producing only what the glass can achieve. Here are plenty of reviews of he 70-300.
FM Reviews - EF 70-300mm f/4-5.6 IS USM. Here are reviews of the 70-200 f.
FM Reviews - EF 70-200mm f/4L USM There is a marked difference in quality. The 70-300 is one of canon's better consumer lenses however it is not quality L glass and does not produce the same quality images.
Remember, bodies will come an go, but top quality glass will last, if taken care of, for years and years.