ATI card &/or drivers problem

Hello,
After removing an old, & inserting a new graphics card & associated software/drivers, my PC won't get past the loading black with progress bar screen. It does startup without the ATI drivers (presumably using Windows default drivers), but when I install/enable the ATI drivers (from supplied CD OR latest from ATI), the screen goes black.
The 6Pin Power supply is connected (tho wasn't needed on previous card).
System;
Fujitsu Siemens Scaleo P 6405
Pentium D 2.8Ghz
Ram: 2048mb, DDR2 667Mhz
'Old' graphics card: ATI Radeon X1550 .
New card: ATI Radeon HD 4850, (Sapphire, dual slot version).
OS: Windows XP Media Center 2005, SP2,
Mobo: Asus P5SD2-FM
PSU: 600w OCZ GameXtreme.
Appreciate any ideas to resolve this,
Thanks,
timtechless.

Problem now resolved (by reinstalling OS, updating BIOS, reinstalling latest ATI drivers.

Good to know, your problem got resolved as I was having the same issue !!!:thumbup1: