Max progressive scan resolution for CGA is 240p. Never used one of those VGA->CGA adapters. But it is $20 bucks. If it sucks, no biggie. Others can chime in if they've ever used one.
You can also set the X360 to 480i and add in an RGB amp. CGA monitors can take a 480i signal. As stated ealier in this thread, it isn't bad. Personally, I have connected a PS2 (@480i) to a NAC with an RGB amp and I can say, it doesn't look bad at all. Never done anything with an X360 however. Only problem is that you will have to build your own RGB amp. I don't recommend the Ultimarc amp as it sucks with contrast and produces a flat image.
You can also set the X360 to 480i and add in an RGB amp. CGA monitors can take a 480i signal. As stated ealier in this thread, it isn't bad. Personally, I have connected a PS2 (@480i) to a NAC with an RGB amp and I can say, it doesn't look bad at all. Never done anything with an X360 however. Only problem is that you will have to build your own RGB amp. I don't recommend the Ultimarc amp as it sucks with contrast and produces a flat image.