http://www.moddiy.com/products/Dell-OptiPlex-3020-PSU-Main-Power-24%252dPin-to-8%252dPin-Adapter-Cable-(30cm).html
So apparently there's an adapter for about $15.
The motherboard is pretty bare bones though. It doesn't even have fan control for the CPU, nor could I find any solid info about audio, SATA performance etc. Does it at least support USB3?
If you get another motherboard instead you might want to ensure your case front USB supports it (some have USB2, USB3 or both).
Examples:
$50 -
http://pcpartpicker.com/part/asus-motherboard-h81me
$70 -
http://pcpartpicker.com/part/asrock-motherboard-h97mpro4
$100 -
http://pcpartpicker.com/part/asrock-motherboard-z97extreme4
$140 -
http://pcpartpicker.com/part/asus-motherboard-z97a
I obviously don't know if you'd want one, but thought I'd give some reasonably good options at different price points. The $70 board looks like a good option to me. My only issue is that aside from the PCIe x16 slot the others are PCI only. I don't know why they did that.
The $50 board has PCIe (can support a dual-slot GPU and single-slot PCIe device). The $140 is overkill in a sense, but it depends on your CPU and how much you're spending on the addon GPU. The Asus Z97-A is a high quality motherboard with a few nice features. If you had a quad-core CPU then it's a system that has a lot of life left.
If you don't have Windows 10 yet, then it might be a good time to upgrade as well.