|If your mboard has a PCI-E X16 slot and you are using a video card in that slot, the minimum PS capacity you need is determined by the video card you are using, or might use in the future.|
You can go to the web site of the manufacturer of the video card model you are using, or might use in the future, and look at it's specifications, and usually you will find the minimum PS wattage capacity a system must have when that card is installed, and more important, the minimum amperage the PS must be able to supply at +12V (el-cheapo PSs often have inferior amperage at +12v ratings, and/or the max ratings are a lie, and/or they can't achieve the rating(s) continuously ) . That's often listed under system requirements.
If you don't see that, any video card with the same video chipset, including any added letters, e.g. X1950 Pro, or XT, or XTX, will have very similar minimum PS requirements.
Sometimes if you merely search the web with the chipset name and something else, you will find that info at the "hits".
e.g. X1950 XT minimum power, or X1950 XT minimum watts.
That said, nobody actually needs a 1000 watt power supply, even if they can install 3 high end PCI-E X16 video cards, and according to what I saw when I was looking at high end PCI-E cards a couple months ago, a decent 600 watt or 650 watt PS, minimum, will handle any current PCI-E X16 card chipset, even if the card is a X2 card (two chipsets on one card), or even if you have two PCI-E X16 cards in two slots.