I just got my Antec VP-450 a dual rail psu with 18 Amps on each rail and I'm very scared to put it into my system.
I bought a 2500k and a XFX HD 6870.
95W + 151 = 246 watts.
How is power distributed on the rails if the rails are rated 18A.
I'm scared cuz I've heard the 6870 can pull 247 watts under max load.
247 / 12 = 20.5A
20.5A > 18A = More Amperes than my psu is rated on each rail.
Does the power the 6870 pull get distributed through the two rails?
I did a calculator and it said that 100% peak wattage would be 418 with 368 recommended which is well under 450. And that's with 100% un realistic loads.
I suggest you read the post i link to from my signature, it addresses why you don't need to worry about a dual rail PSU.
As for your questions, Guru 3D's review of the 6870 had the whole system pulling 295 W from the wall under full load, this is the combined power draw of the CPU and the GPU both maxed out, a 6870 only has a TDP of ~150 W so that is about how much it will draw. http://www.guru3d.com/article/radeon-hd-6850-6870-revie...
The power supply will have the cables distributed in a manner that will make it difficult to overload a single rail, according to hardware secret's review of that particular model the CPU power cable is on the second rail, all other lines connect to the first rail, this means your CPU is powered from one of the rails and the graphics card along with the rest of the system is fed from the other, this is less than ideal but because the actual OCP limits are set slightly higher than those on the label(OCP isn't a precise science it has some variance) you won't have a problem with overloading a single rail and shutting down the system since you are only really dealing with about 300 W of load and 100 W of that is for the CPU on one rail, and some of the rest of that is on the 5V and 3.3V rail. http://www.hardwaresecrets.com/article/Antec-VP450-Powe...
You never specified the exact XFX model number of your card but here is an example for an XFX card.
A XFX Radeon HD 6870 2 GB GDDR5 with DD-Cooler (HD-687A-CDFC) has the following true measured power consumption (i.e. total of +3.3V and +12V power lines of the PCIe x16 slot and from both PCIe Supplementary Power Connectors):
Furmark: 184.22 Watts
Gaming: 151.98 Watts
With the card Overclocked to 970 MHz GPU and 1210 MHz Memory:
Furmark value not given. Most likely due to self-preservation throttling kicking in.
Gaming: 166.26 Watts