To connect my Hantarex Polostar arcade monitor to my consoles I use the
Micomsoft XSelect D4. This neat little box has multiple inputs selectable by remote control. It can transcode between VGA and component. It is perfect for arcade monitor usage as 525i is transcoded to 15kHz RGBHV, while 525p is trancoded to 31kHz RGBHV. This means it does not do upscanning, so the display will always look authentic. And trust me, you don't want the blurring and color bleed associated with upscanning.
Inputs:
1 x VGA (31kHz only)
3 x component video
1 x 21-pin RGB (15kHz only)
outputs:
1 x VGA (15kHz for 525i and 31kHz for 525p)
1 x component video
Someone mentioned the upscanning solution. I would stay away from those $50 upscanning boxes. They are low-quality devices that dispay a lot of quality bleed and at best have S-video inputs. I was not impressed with them.