Yes that is true. If you go for something too fancy then you will need to use a 6 pin power cable from the power suply to help power the card. With my old 9800 GT there was no need for this external power, it all ran from the power on the PCI-E slot. Now I have my GTX 460 I use the relevent power cables to give the card the extra power it needs.
Also you need to have enough power to power the whole machine. If you bought it from a shop already built then your more than likely going to have a fairly small amount of power coming from the power supply (Just enough to power the system). The output on the power supply is measured in Watts. A card that Does not use any exrta power from the PSU can be about 100W + of power whereas a card that does need extra power will need more. You can visit this website Thermaltake Power Supply Calculator
to get an estimate on how much power your system is using and how much it would be if you upgrade your graphics. Use you stats to help Stats
On top of that you have the Amps and the 12v rail requirements however if you have a decent brand of power supply then your dont normally have to worry about this.
Im guessing that yours is one that was in the system when you got it. Take a look and report back the Watts and the brand of your PSU.