I remember having similar trouble when I upgraded to KDE4. Have you tried removing/renaming your .kde directory? (That's assuming you're upgrading from KDE3.)
- [gentoo-user] startx fails launching KDE 4 Andrey Vul
- Re: [gentoo-user] startx fails launching KDE 4 Hilco Wijbenga
- [gentoo-user] Re: startx fails launching KDE 4 Harry Putnam
- Re: [gentoo-user] Re: startx fails launching KDE 4 Andrey Vul
- [gentoo-user] Re: startx fails launching KDE ... Harry Putnam
- Re: [gentoo-user] Re: startx fails launchi... Andrey Vul
- [gentoo-user] Re: startx fails launc... Harry Putnam
- [gentoo-user] Re: startx fails l... Harry Putnam
- Re: [gentoo-user] Re: startx ... Andrey Vul