Both DVI and VGA are being phased out on newer monitors. You will need to purchase a DVI to HDMI cable. The problem is most older video card HDMI out is version 1.3. The monitor HDMI in is version 1.4. Your video card was released in 2010 and uses version 1.3. The maximum resolution you will be able to get is 2048x1152 60Hz. It is not economically wise to purchase the video card DVI to monitor DP/mDP active converters.

Thanks for the response Chris.

We use the native resolution of 1920x1200 on these displays, so I don't think that will be a problem.  I think going forward, we'll just switch video cards to one that has 2 or more DP ports, but for the older systems the DVI to HDMI cable should work.

I'll order one and try it out. 

Cool. A little confusion on my part, I thought you meant the U2515H (2560x1440 60Hz), but you meant the U2415 (1920x1200 60Hz).

Chris - I have this same problem, only I just bought the U2515H monitors. My OptiPlex 790/with video card was purchased 7/2011. Would it be economically wise to purchase the video card DVI to monitor DP/mDP active converter for my one computer? Can you give me this part number?

Thank you!

It is not a Dell product so no Dell part number. Here is one.

Chris, The Dell salesperson that I have been working with has just suggested that I need Item #330-6841.

Do you think this will work?

Thank you!

I have not tested it. But for $19.99, it may be worth it to you to test. You can always return it.