That's a link to a GeForce GT 610 (Retail) card not a GeForce GT 620 as you've mistyped in your first sentence.
The GeForce GT 610 is just a rebranded GeForce GT 520.
Make sure your power supply has a combined +12 Volt continuous current rating of 18 Amps or greater.
You didn't specify exactly which Dell OptiPlex GX280 model you have. The power supply unit capacity is different for each form factor so your system's power supply may or may not have the capacity to support the card.
Small Form-Factor: 160 Watts
Small Desktop: 210 Watts
Desktop: 280 Watts
Small Mini-Tower: for processor speeds of 3.2 GHz or lower: 250 W; for pocessor speeds of 3.4 GHz or higher: 305 Watts