sci-chemistry/sparky-3.113 doesn't work with Python 2.5 according to dependencies specified in the ebuild.
This might be better in sparky-3.115, which has been released in May.
I will take a look into it.
I did the version bump in the sci overlay: version bump to 3.115 as-needed respecting PREFIX aware (whitelist ready) multilib aware No fixed python and tk/tcl versions anymore
+*sparky-3.115 (13 Feb 2010) + + 13 Feb 2010; Justin Lecher (jlec) <jlec@gentoo.org> + +files/3.115-fpic.patch, +files/3.115-ldflags.patch, + +files/3.115-makefile.patch, +files/3.115-paths.patch, + -sparky-3.113.ebuild, +files/3.115-wrapper.patch, +sparky-3.115.ebuild, + metadata.xml: + version bump, removed fixed tcl/tk and python version, multilib awareness + added, EPREFIX ready, keyworded for ~amd64, bug 267355 299348