Similar situation:
When I use fglrx with default configuration it blanks the screen and hard locks the system (VTs and network die). I have to disable AIGLX, Composite, Accleration and DRI for it to work at all. If I change VTs it crashes.

I don't have a BIOS option for Memory Re-Mapping so I can't test turning it on/off. I haven't yet tried removing/replacing a RAM stick.

Hardware:
Intel Core 2 Duo E8400 (45nm, 3.0GHz)
2x2GB = 4GB DDR2-800 Kingston
Gigabyte EP35-SD3R motherboard
Sapphire ATI Radeon HD 2400 PRO

sj26@viridis:~/src$ lsb_release -rd
Description:	Ubuntu 8.04
Release:	8.04

sj26@viridis:~/src$ uname -a
Linux viridis 2.6.24-18-generic #1 SMP Wed May 28 19:28:38 UTC 2008 x86_64 GNU/Linux

sj26@viridis:~/src$ cat /proc/meminfo 
MemTotal:      4053800 kB
MemFree:       3065532 kB
Buffers:        589464 kB
Cached:         217388 kB
SwapCached:          0 kB
Active:         173052 kB
Inactive:       731264 kB
SwapTotal:     1526132 kB
SwapFree:      1526132 kB
Dirty:              12 kB
Writeback:           0 kB
AnonPages:       97472 kB
Mapped:          32640 kB
Slab:            41824 kB
SReclaimable:    24392 kB
SUnreclaim:      17432 kB
PageTables:       7028 kB
NFS_Unstable:        0 kB
Bounce:              0 kB
CommitLimit:   3553032 kB
Committed_AS:   330552 kB
VmallocTotal: 34359738367 kB
VmallocUsed:     20608 kB
VmallocChunk: 34359717375 kB

sj26@viridis:~/src$ cat /proc/mtrr 
reg00: base=0x00000000 (   0MB), size=4096MB: write-back, count=1
reg01: base=0xe0000000 (3584MB), size= 512MB: uncachable, count=1
reg02: base=0xd0000000 (3328MB), size= 256MB: uncachable, count=1
reg03: base=0x100000000 (4096MB), size=1024MB: write-back, count=1
reg04: base=0x130000000 (4864MB), size= 256MB: uncachable, count=1
reg05: base=0xcff00000 (3327MB), size=   1MB: uncachable, count=1

For the open-source radeonhd driver which sort-of works I got the same/similar message as my current fglrx setup in the Xorg log:

(WW) fglrx(0): Failed to set up write-combining range (0xd0000000,0x10000000)

I'm using fglrx from the ubuntu restricted drivers package:
sj26@viridis:~/src$ apt-cache show xorg-driver-fglrx
Package: xorg-driver-fglrx
...
Source: linux-restricted-modules-2.6.24 (2.6.24.13-19.42)
Version: 1:7.1.0-8-3+2.6.24.13-19.42
...
MD5sum: d25fed1486a3a70766969a54b24b7800
SHA1: bffc8ce322c7127cbbe443254e517fb3e7b8cf8b
SHA256: d54494b5003acc245c988dcd3573b67316a4f7a685380e3661ef847ff6e09ec3

/etc/X11/xorg.conf:

Section "ServerLayout"
	Identifier     "Default Layout"
	Screen      0  "aticonfig-Screen[0]" 0 0
	InputDevice    "Keyboard0" "CoreKeyboard"
EndSection

Section "Files"
	RgbPath      "/etc/X11/rgb"
	ModulePath   "/usr/lib/xorg/modules"
	FontPath     "/usr/share/fonts/X11/misc"
	FontPath     "/usr/share/fonts/X11/100dpi/:unscaled"
	FontPath     "/usr/share/fonts/X11/75dpi/:unscaled"
	FontPath     "/usr/share/fonts/X11/Type1"
	FontPath     "/usr/share/fonts/X11/100dpi"
	FontPath     "/usr/share/fonts/X11/75dpi"
EndSection

Section "Module"
EndSection

Section "InputDevice"
	Identifier  "Keyboard0"
	Driver      "kbd"
EndSection

Section "Monitor"
	Identifier  "aticonfig-Monitor[0]"
	Option	    "VendorName" "ATI Proprietary Driver"
	Option	    "ModelName" "Generic Autodetecting Monitor"
	Option	    "DPMS" "true"
EndSection

Section "Device"
	Identifier  "aticonfig-Device[0]"
	Driver      "fglrx"
	Option      "NoAccel"
	Option      "NoDRI"
EndSection

Section "Screen"
	Identifier "aticonfig-Screen[0]"
	Device     "aticonfig-Device[0]"
	Monitor    "aticonfig-Monitor[0]"
	DefaultDepth     24
	SubSection "Display"
		Viewport   0 0
		Depth     24
		Modes    "1920x1080"
	EndSubSection
EndSection

Section "Extensions"
	Option "Composite" "disable"
	Option "AIGLX" "disable"
EndSection