Motherboard issues using several GPU's

I was wondering if some of you hardware guys could help me on a little issue I have. Since TBC release I've been hardware multiboxing, but now recently I have gone software boxing to save heat,power and last but not least, space in my office.
I have a lot of monitors in my rig, however I only use my 30" and one 20" for boxing. However I would like several more outputs to work correctly since I also has a 42 TV and other monitors for other purposes on my system.

However I am having some issues with my motherboard/gfx cards.

Hardware I am using that may be relevant to my issues:
Corsair Powersupply 750W Black, ATX/EPS
Intel Core 2 Duo E6850 3.0GHz 1333Mhz
Asus P5N-D, nForce-750i SLI, Socket-775,
2x Corsair TWIN2X 6400 DDR2, 4096MB CL5
XFX GeForce 8800GTX 600M 768MB XT GDDR3 (primary card,PCI-E)
Gainward GeForce 6800GT 256MB 2ns GDDR3 (secondary card, PCI-E)
2x XFX GeForce 6200A 512MB DDR2 ( 3rd and 4th card, PCI)
30" Dell 3007wpf (2560x1600) (on primary card)
20" on each side vertical (1200x1600) (one on primary card and one on secondary card,because of my issues,the one i use for boxing is on the secondary output on my primary card)
misc other monitors on the remaining adapters

The problems:
- When I am using several video cards, my computer will not boot if it is cold ( when booting vista x64, it seems that the display driver will not start. However if I try again after 5-10 mins when the comp has warmed up, it boots fine.)
- All but primary videocard is sluggish. It seems like acceleration only works on my primary card. 3d is useless on the other cards, and even using a browser is sluggish (like when you have no driver installed, and using windows standard driver)
- If I change any settings, all my displays will flicker a lot.

What I have tried and results:
Removing all but primary cards - When I did this, everything seems to work perfectly. Computer also boots up fine when it is cold.
Removing all but primary and secondary cards - Same problem as with all four cards.
Flashed motherboard bios - No improvement

I have read somewhere that not all motherboards is very happy with using several videocards. Is this true, and if so, any suggestion on wich motherboard I should use so I can have 5+ monitors?

Also, is using several different model gfx cards a problem? I've also read that all 6xxx and newer nvidia cards use the same driver, but you cant mix them with 5xxx and older cards. But since all my cards are 6xxx and newer, this should not be a problem (?)

Any help would be greatly apreciated :)