That word "impress" scares me!
I think the mashed potatoes would be fine.
I usually do lamb, roasted potatoes and asparagus or peas with minced red onion and a salad.
I would stick with things you make on a regular basis, that can be done ahead and that require very little last minute fussing. I would also encourage you to find a girl that enjoys cooking with you and not one that enjoys being entertained or impressed. Open the wine, you can fix the lamb chops while she makes the salad etc... That paints a much nicer picture of the next fifty years than you fussing around in the kitchen while your wife sits in the living room waiting for dinner to be served!
Good luck and have fun!