Ok, I have a 9800GTX gfx card, and I need a power supply. The card's Maximum power is 140 W, recommended minimum power supply is 450 W. My question is what is the minimum amperage (on the +12v rail) to run this card. I read somewhere saying it was 24A, however that was for a single rail, if a PSU has 2 12v rails @ 18a each, would the card still work.
The 24amp rating is for high-end cards that can suck about 200-250w of power. 18amps is fine.
the GTX 9800 requires a rail that provide 20 - 30 amps of power, and suggests a psu of 450W. But yes, if the graphics card as two inputs and your psu is setup to split the power between the rails then yes, as long as the power is coming from both rails. However if say your gpu only has 1 8 pin input or all pci-e power was on the same rail then it would not work.
@jerm1027
It is a high end card, in its time it would have been equivalent to a GTX X80 card.
I never take into account the whole system, my bad, but my conclusion wasn't wrong. Assuming the rest of the system takes 250w, that makes close to 400w with the GPU added, divided by 12v, means you would need about 33 amps total of power. Again a dual 12v @ 18 amp would do you fine, but you need to makes sure it spreads the load the evenly, especially if it has more than 1 PCI-e connector.
I understand the 9800GTX was a high-end card for the time, but keep the time in perspective. Gaming was still very CPU bound, and GPU's didn't need to be as fast, and thus weren't as power hungry as today's GPU's. The power consumption of a 9800GTX isn't anywhere near a 580, nor a 780, both of which have a TDP rating of 250w. The TDP for a 9800GTX was 140w. I still remember seeing a few single-slot coolers.
This is a GeForce 6800 GT, also a very high-end card for it's time:
http://forum-en.msi.com/faq/article/power-requirements-for-graphics-cards
Here is a good list of Graphics Cards, and power requirements for reference.