The 250 ohm Beyerdynamic DT 990 Pro headphones can be used with the sound card of your ROG Strix 490 F, but there are a few points to consider:

- Most laptops and PCs can power 250 ohm headphones, although the audio output power may vary depending on the computer model. In general, tests have shown that many laptops and PCs can provide sufficient volume for these headphones, although you may have to adjust the volume to 60-75% to get a comfortable listening level.

- It is not mandatory to buy a dedicated sound card, but it might be useful if you want more power and better sound quality. An external sound card might offer better dynamics and sound fidelity, especially if you work with high-quality audio.

- The 80 ohm DT 770 Pro headphones require less power to reach the same volume level, so they might be easier for your integrated sound card to handle. However, if you prefer the sound and features of the DT 990 Pro, there is no need to switch to the DT 770 Pro for that reason alone.

In summary, the 250 ohm DT 990 Pro can work with your integrated sound card, but a dedicated sound card could improve the sound quality and power.