The U stands for unbuffered. The unbuffered will work but this forum post lists the RAM limitations. And the RAM you posted is 800MHz while 400MHz is certified for the 280. If you install the PC-2-6400 instead of PC2-3200 it will probably run at 400 MHz. However this site sells PC2-6400 RAM for your system which would indicate that it will work.

Keep in mind that the 280 is pushing 15 years old and almost anything you do will not greatly improve performance on such an old system. Maybe time to consider an upgrade to a new system.

6400 ram may not work due to being high density and spd wait states being too high.

https://www.amazon.com/PC2-5300-DESKTOP-Modules-240-pin-Tech/dp/B00C53831I/

Max Ram is 4 gigs minus the ram stolen for intel video.

GX280 is well past end of life.

GX280 uses LOW Density Ram.

https://www.crucial.com/usa/en/compatible-upgrade-for/Dell/optiplex-gx280-%28desktop-and-mini-tower-models%29

EDIT Crucial has stopped making ram for this model.