Upgrading computer - now no boot, please help

Hi all :)

I think I've done something wrong that might have messed up my computer and I'll be very grateful if you can help.

I've tried to make some hardware changes in my computer. I changed the graphic card to a crossfire system, added a new hard drive, add 4 GB RAM and removed my old floppy drive. Then I connect it with a display, and it starts and I can hear that it searches for windows on first the disk drive and then the hard drive but I get no output on the screen... Okay, then I removed the new graphic cards and put the old one back in. Still no output... Then I changed it all back, to what it was, and still no output? What could be wrong, if you have any idea please tell me ?
Maybe there is something i can do to rule out certain components?
I probably should mention that the display I was testing it on were not my standard display, but an old Fujitsu-Siemens display, but still a VGA flat screen. I tested the display by connecting it to my laptop and it worked fine.
Old specs:
MSI K9A2 motherboard
AMD Athlon 64 X2 6000+ 3,1 GHZ
Sapphire Radeon 3850 512 MB
4 GB RAM Kingston
Maxfactor(or something) harddrive 250 GB
Harddrive 80 GB
A DVD, a CD and a floppy drive.
New specs:
MSI K9A2 motherboard
AMD Athlon 64 X2 6000+ 3,1 GHZ
2 x Sapphire Radeon 6950 dirt edition in crossfire
8 GB RAM Kingston (not exactly the same as the old ones, but they fit ? )
No floppy drive and added a 1 TB Seagate Barracuda 7200.12
Hope you can help!! :) Thanks in advance!