You basically need a PSU, with two 12V rails. The 2 rails does not mean two physical 12V molex/pcix connectors. It just means that internally, the PSU can dedicate two 12v signals each with high amps. This is split into a.) CPU and b.) the rest of your system.
Seeing that amps wise, the gcard will consume much more than the rest of the system, it falls on this rail. The CPU requires a clean signal, and seeing that the PCIx x16 slot runs off the 12v from the connection to the mobo (hence the cpu - partially actually) it makes sense to let the other rail go to the mobo and cpu.
CPU/Mobo 12v rail -> 80% CPU / 20% mobo
System 12v rail -> 80% Gcard / 20% system
In effect, the gcard gets it's power from both rails, but more from the system 12v rail.
To answer your question, the 1900xt will be fed externally through the PCIx power connection found on all ATX 2.x psu's. This 12v falls under the system 12v rail. Alternatively, there will be a connector with the gcard, that converts 12V via molex to pcix power connection.
thanks for the replies, I guess what I'm trying to ask exactly is do I need to purchase a Y connector to connect two 4pin plugs into a single 6pin(?) plug? I plan on purchasing an Antec 550 PSU with two 12v 19a rails. does this psu come with 6pin(?) connectors to go straight into the x1900xt? or do I need to connect the two 12v rails with that Y connector? I am not sure how all the psu stuff works since I never had a video card that really required high power requirements. thanks.
well, I not sure what im talking about If I buy a x1900xt and an antec 550 PSU, will there be a special connector on the PSU to fully power that vid card ? or would I have to get some kind of Y connector to connect two 4 pin connectors?
sorry if this if my questions arent very clear, I'm having a hard time conceptualizing how a PSU works with volts and connectors and what not.
Yes, if it's a Antec TruPower 2 - it will have a six pin PCIx power connector. The gcard will ship with a y connector, with two 4pins molex going to the 6 pin - but it's not to power two 12v into the 6 pin, the one molex (4pin) is male and the other female, so it allows you to power from the psu to the 4pin, which then goes to the 6pin pcix, and then from the last 4pin you can connect a hdd/cdrom/fan etc (series connection).
For higher end setups like xtx crossfire or 7900 gtx sli setups and configurations of that sort, newer motherboards provide for more power stability by adding a 4 pin molex connector directly ON the motherboard (in addition to the 4-6 or 8 pin power connector and the 20 or 24 pin power connector boards normally have).