What do you mean "external graphics card?" Almost all desktop computers run on an internal graphics card which you plug in to a pci-e slot on your motherboard. There are external eSATA cards for enthusiast level notebooks. The cheapest modern card which runs adequately for most tasks is the amd 6670. You could also run a 7750 for a little more gra power, or spend up to around $250 on any of the other gtx 600 or amd 7800 level cards and get even better performance while sucking more power.
The low end gpus don't need power adapters but better ones always do.
If you put in a 6670 and plug your monitor into the new graphics card everything should work.
for instance kevin, i have tried a gpu of 128mb to check display in pci-e slot, but its not working either. will i be needing a higher end card?