I upgraded kernel to 2.6.15 and after re-compiling nvidia proprietary kernel module (version 1.0.6629-r5 - gentoo), X.org gave me following error:

X Window System Version 6.8.2
Release Date: 9 February 2005
X Protocol Version 11, Revision 0, Release 6.8.2
Build Operating System: Linux 2.6.14-ck6 i686 [ELF]
Current Operating System: Linux cloudkiss 2.6.15 #1 PREEMPT Sun Jan 8 09:18:28 EET 2006 i686
Build Date: 09 January 2006
Before reporting problems, check http://wiki.X.Org
to make sure that you have the latest version.
Module Loader present
Markers: (--) probed, (**) from config file, (==) default setting,
(++) from command line, (!!) notice, (II) informational,
(WW) warning, (EE) error, (NI) not implemented, (??) unknown.
(==) Log file: "/var/log/Xorg.0.log", Time: Thu Jan 12 09:31:55 2006
(==) Using config file: "/etc/X11/xorg.conf"
Using vt 7
NVIDIA: failed to set MTRR 0xd0000000, 256M (write-combining)
XIO: fatal IO error 104 (Connection reset by peer) on X server ":0.0"
after 0 requests (0 known processed) with 0 events remaining.

I couldn't make it run, so i downgraded to 2.6.14 and everything was back to normal. Any ideas what has changed in kernel causing such error?

CPU: AMD Athlon XP 2200+
MEM: 768M
GFX: GeForce 6600GT