Amps need for my 9800GTX+

no

single rail have unlimited power, quad rail is limited and most are cross conntect

v1 + V2 +v3 = v1

v1 + v2 +v4 = v2 and so on

if you have a quad rail, the max is 12v and most have a max of 18amp or 150watt some go up to 24w but that rare and may only be peak power

if you 9800GTX usess up to 250w
then the first 75w comes from the pci-e slot
then the next 150w comes from the first 6 pin - first on how its wired not in order you look at

the remaining is from the second 6 pin

8pin gives more and cross conected for hiigher amp on single rail

4870X2 can suck up to 350w+

i do not want to mislead you
the logic of each card and equally pull from both plugs or can order them

so the 75w comes from the pci-e slo - this is the max for the slot - this limit for the mobo traces

the rest can be split equally from both

75w-250w (say this is the~ max) = 175/2 = ~78w/12v = ~6 amp

6 pin PCI-e connector can only provide 75W max.

This means at least 24A total available on the 12V rail(s).

  Thank you all that responded, that was the clearest and most straight forward answer I have seen in any forum. I fully understand and really appreciate your knowledge.  

U need minimum 2 lines of 12v ...

The 9800GTX+ needs about 9 amps. The 24 amps is the total recommended power for a "typical" - whatever that is - system.

Yes, it's true. I have a HD3870 105 Watt Video Card - I had on it 400W 12v - 18amps - worked!

You need 2 PCI-e 6 pin connectors, but the number of 12V rails does not matter.