for #1 basically by googling "x1950 problem power"... it applies to agp and pci-e as well since they are the same card except for the interface and a bridge chip..
http://www.ricklafay.com/pc_power.htm "someone came up with the conclusion that we needed a minimum of 30a on the 12v rail" lots of other good quotes, his card is x1950 is agp.
http://www.google.com/search?hl=en&safe...22+30A+12+volt+recommended&btnG=Search
http://www.hisdigital.com/html/faq.htm -randomly for different x1950s
http://techreport.com/forums/viewtopic....5&sid=89aa3cdc380d2ba4bef570c8b9705e81
for #2 single rail is better because its more efficient, also less worry on the installer's part since they dont need to worry about evenly distributing stuff across the rails.. and the highest end GPU's basically will need it if under full load for a long time. with a single rail, there would not be any fluctuation in the 12v voltage, unless the multi rail is evenly distributed for the devices, this WILL happen, and can cause issues.
here is some propaganda.
http://www.tomshardware.com/2005/07/11/stress_test/ shows a couple power supplies that failed from load.
http://www.madshrimps.be/?action=getarticle&articID=511 is a really good review, basically showing single rail at it's best.
http://www.ocia.net/reviews/precise850/page2.shtml talk a bit about single vs multi rail.
http://www.pcpower.com/technology/myths/ #8, its directly at the single rail vs multi.
http://www.bjorn3d.com/read.php?cID=1048 "Yet later down the road we learn that a single-rail power supply unit not only offers more stable and efficient power if designed and built correctly; it would also cost less."
http://forum.msi.com.tw/index.php?topic=77071.msg543206 has a few interesting posts about multi rail. such as 1 important concept. a dedicated rail to the cpu, which of course now adays is moving toward performance per watt. but looking at a GPU, they would need multiple rails connected directly to it. and with multi rail power supplies its not really doing this, especially if connected to hardware and a video card wrong by using the same line on the video card and other things, pci-e seems to get away from this issue at least.
edit- adding that i wasted 20 hrs after purchasing an sapphire x1950 agp while i waited for it to arrive from zip zoom doing more research on the card once i found out people had problems with that card. after 20 hrs of my own research, i found exactly what rick lafay found. and that was that most people with a "dead" x1950 sapphire agp had a weak power supply since it was not really 30A+ on the 12V rail..There were cases in the research that showed just bad cards that were DOA, etc. but for the most part it was power, then heat, then drivers on the x1950.. mostly power problems for people out there. some cards died instantly on boot. others died from the load over time since the power supplies basically killed the cards. if someone researches the geforce 8800 power requirements, they will find very similar requirements, but it doesn't explicitly say single rail on either ati's or nividia's. its something that doesn't really show in reviews either since a benchmarker doesn't stress it long enough for the cards to have a problem since benchmarks are quicker with breaks in between benchs. if you or i, or anyone else ran an x1950 or an 8800, etc. for 24 or even 48 hours straight or longer under the most intense game available nonstop. the weaker power supplies will die, or a random problem like a reboot or a crash will occur that boils down to power issues instead of something else like bad programming of a driver or game. a few rare reviews on the web that really stress a pc long enough will throw in some huge wattage psu in and get problems with it since its multi rail, but this really tough for me to find one to link since its really rare to find a review that is long enough to find problems with the item that is being reviewed.. essentially googling for dead x1950s is quickest and gives very similar results of people thinking any power supply, even multi rail is perfect or good enough. for some people it does work, but looking at the forums, there is more out there that have issues than those without it when using a power supply with not enough power or to an extent, multi rail with a bunch of 18A on the 12v that has a LOW combined rating that is less than 30A. the ones that don't have the issue probably have a great combined max for the 12v that is over 30A when they use a power hungry video card, or use several power supplies to get the wanted results. they could just be down right lucky or not stress the pc enough for long enough to see the power problems.