rolli59 :
Maybe! It depends on the quality of the unit. Most of those cards do not need power from the PSU so they will run on modern 300watts.
Check your bios. Often enough some mobo's don't automatically switch to PCI as the adapter. Also try every connector on your card and plug them in and see if they display anything. If not, remove the card and reseat it on the mobo (sometimes its just bad signals getting jammed).
Often enough to me that display don't work is not due to faulty cards but mostly it has something to do with windows and or drivers. Sometimes windows OS just do not like making your display device work properly. Because my computer does that using a DVI connection. When the comp goes to sleep and turns back on the display is blank. But when I use HDMI no problems at all. My DVI cables are all fine and the card is not faulty. Just one of those stupid things.
300W is more than enough, low-end cards do not consume lot of power.