Doesn't matter if a PSU can deliver 5000 watt if it can't deliver more than 10 amps on the 12V, the voltage the video cards need. Watt is voltage*current, so the wattage your PSU can deliver to the video card is entirely dependent on how many amps it can push down the 12V.
Ideally, a good PSU should be able to push most of its rated wattage to the 12V rail(s), and second most important, the 3.3V, where the CPU gets its power. These are the two most power hungry components in the PC.
PSUs made with powerful video cards in mind also sometimes have several 12V rails. My relatively weak PSU (in terms of wattage (450W)) can still power most new cards because it's got two 12Vs, one that can deliver 15 A and another that can deliver 16 A. It's also able to let almost all the total power be directed to those two rails. A 500W PSU with just 8 or 10 A on 12V would be destroyed if trying to run video cards my PSU has no real problems with. Not that 500W PSUs would ever be configured like that in this day and age.
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