An aluminum shroud won't help unless your cardboard one is leaking air, or the new is a better design. The material itself won't effect performance.
A larger radiator would be able to tranfer more heat, but does your system need it? The Chevette core is pretty efficient and should be more than enough for most systems. Running dual CPUs, peltiers, etc. is a whole differant ballgame, though. If you have a "typical" seet-up, chances are you have other, weaker links than the rad. Tell us more about your rig for a better answer, though.
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-Mickey
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