Monitor never starts up with new computer

serious problems for me here.
my screen is all black on my Dell M992 when i start windows XP.
i have just bought the computer and i tested it at the guy that sold it to me, it worked with his monitor (LCD).
i have an old one, Dell M992 and hoped it would still
work with my new computer - i am pretty poor nowadays  

i hear one short beep while starting up and
even when the monitor is not plug in (tested that too)
AWARD BIOS beep codes says that one beep is a successfull startup.
what can be wrong? is it too old for the graphics card?
my monitor is blinking during the whole process.
and i connect it with a D-Sub on motherboard and take power with a other cable. im surfing with my old Dell machine right now.

have searched the net for a same problem but could not find one.
i don't know what to do when it's all dark, hard to trouble shoot for me.

setup:

Dell M992 (Monitor)
Chieftec Smart Micro-ATX Tower
Intel Core™ 2 Duo E8400 3.0GHz
PowerColor Radeon HD 4850 "PCS" 1GB, GDDR3
Corsiar XMS2 4GB DDR2 Matched
Gigabyte GA-EG41MF-S2H Motherboard
DVD-RW
Coolermaster 650W
Samsung 500GB S-ata2 Harddrive

sorry for bad english