I'm looking for a restaurant with a party room for about 40 guests for a small wedding. A garden would be a great addition.
You don't say what cuisine(s) you prefer or what kind of budgetary constraints you might have. That said...
Gascogne, the small French bistro, on 8th Av., b/t 17th & 18th Sts., has very good food, a charming interior, and a truly lovely back garden. They do private parties.
We got married at Moran's in Chelsea after recomendations from Chowhound. Tom the banquet manager is a gem, the food was great the service excelent, and did I ad the banquet staff makes home made cookies for the tables? They weer very amenable to working within our budget too.