Well, I know for a fact Windows XP doesn't have a hard 4 GPU limit. I used to run Folding@Home on 4 9800GX2 video cards, so 8 GPUs, on Windows XP x64. If there is an issue, it's manufacturer specific then.
I tried to mine with 8 GPU's in Linux - Ubuntu 11.04. Mother board is MSI big bang Marshal. The problem is not Linux but PSU.
I can mine with 8 cards , if don't over clock, don't have any fans installed....
To cool GPU's i use 120 mm fans, 6- 8 fans & PSU can't able to give power with 8 cards + fans.
I used 2 PSU's & i still can't believe the PSU sucked.
Cooler Master Silent Pro Gold 1200 W. Before buying i wrote to cooler master & got confirmation that it will support 8 GPU's.
Yes it supports 8 GPU's in Ubuntu, but mining & adding fans makes the PSU to go off.
Then i used another PSU, 3, 1200 W to mine with 8 cards, but due to not able to under clock memory of 6970's heat went up to 95C+ , so stopped using that.
My advice, unless you know how to cool, avoid more than 6 GPU's.
Also Cooler Master even though having 80 gold certificate sucks, seems they got fraud certificate or something & also in AMD's certified PSU's only 2 nos of 5870 is mentioned as CMSPG 1200w supports.
If you want more power, yesterday found out, silverstone ST-1500W can able to with stand 4 GPU's , I emailed them & they confirmed.
Still cooler master gave me failure , i can't trust it now. But 1500W can support, i hope.
My system had 5 nos. of 5870's (2 ASUS EAH 5870, 1 Sapphire HD 5870, 2 MSI R5870 Lightning) & 3 nos. of MSI R6970 Lightning.
Well duh, of course a single 1200W power supply isn't going to be able to handle the load of 8 GPUs. You get a max of 98A @ 12V, so 1176W to work with on the 12V line. Figure your system probably has around 100W overhead, so you're down to 1076W to split among 8 video cards, or 134W each. Unless you're using some pretty low performance video cards, you're overloading the power supply. You're probably going to have to run a second power supply to power at least a couple of those cards.