饭馆 sounds more casual ,as long as it has desk and chair(sometimes even desk can be needless). like this: http://static3.cn.openrice.com/UserPhoto/photo/1/1HM/00ALDUE1792F8A6EF89AB0l.jpg 餐馆 sounds a little bit more formal than 饭馆，it's more common on map and phonebooks.usually we dont use 餐馆 in conversation. it usually like this: http://pic19.nipic.com/20120317/9617254_212715261189_2.jpg for fancier place,we use 餐厅. but in conversation,we use 地方（place) or the name of the restruant to call it. for the second question,在 means on/at,if you say 我们在餐馆吃饭,it means you are eating in there.but if you say 我们到餐馆吃饭,it means we go there and eat.到 means to/arrive.

"饭馆" and "餐馆" have same meaning. In our regular life,we say "饭馆" more.

both are correct, just different meaning 我们 一起 在 一家 中国 餐馆 吃饭。 = We will eat at a Chinese restaurant together. 我们 一起 到 一家 中国 餐馆 吃饭。 = We will go to a Chinese restaurant to eat together.

at a restaurant 只是“在饭店（里）“、”在饭馆（里）“、“在餐馆（里）”的意思，根据情况来决定用不用”里”。 而如果没有给出明确的意思，单看”我们一起（　　）一家中国餐馆吃饭。”的话，用“在”和用“到”都可以，只是意思不一样。