I would start with what you have for a router, is it an older G band router or is it a newer b/g/n/ router or b/g/n dual band or simultaneous dual band. That goes a long way towards determining what to get. You also have to know what the other wireless devices have for adapters because if you don't have simultaneous dual band then the slowest G band adapter will control the speed since the router will have to transmit tio that G band adapter.
If it's at all possible you can see if you can run an Ethernet cable to the room with your computer. Don't say it's impossible because it isn't , you may not know how to do it but I can assure you that it can be done and it may be worth it for you to hire someone to do this just like you would pay for a good router or an expensive adapter it mght not cost thatmuch and it would be worth finding out what it woulsd cost. There is no better connection then a wired connection.