After the compiler is installed im going to recompile glibc using gcc 4.0 and recompile other major parts of the system
This is probably more trouble than it is worth. gcc-4 won't be fully ABI compatible with gcc-3.4, and you are likely to find bugs in gcc and various other packages, as gcc-4 hasn't been used to build a full distro yet. How much trouble you see will depend on your target, which you didn't specify.
Unless you are in the business of making distros, or want to learn how to build a distro, it is best to stick with the compiler that your existing distro supports.
Also, it is probably a little early to worry about this, as the gcc-4 release is probably still 2 months away.
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Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com
