as you can see in the Minimum System Requirements in that page says "A minimum 300W system power supply"
The link that you provided is dead, but assuming that you found that 300 watts is fine, just consider what box you have. If this is a tower, any card in this series should fit. If it is a slimline Optiplex, then you will need a low profile video card rated at 300 watts or less.
Also consider what hardware is in the system. If you have a stock Optiplex with no added hardware (added more hard drives or other internal hardware that may draw on the power supply), this probably will be fine. However, if you have added internal hardware it will affect how much power is available to run the video card that you choose.
I see that you posted on the Dell forums, which is your best bet for an answer, since their motherboards are often proprietary and have specific specifications. What you want is someone who has added an external card to the system and knows that it works. Your more likely to get good answer on Dell's forum specific for your system, unless someone here has tried it on the same or like system.