(Please bare in mind that I am still fairly new to XNA, so if there is a better / more efficient way to accomplish this, please disregard my advice ;) )
To elaborate more on what RanBlade described:
instead of creating two new integers, XNA already stores the size of the screen for you inside "graphics.GraphicsDevice.Viewport.Width" and "graphics.GraphicsDevice.Viewport.Height"
As for the win condition:
create a bool variable called hasWon, and a bool method called DetermineVictory(). Inside, have it test whether or not either players score is equal to the score To Win. Set hasWon to true or false based on the result, and have the method return true if either player has the score to win.
Then, inside your Update(), have a do/while loop that checks if DetermineVictory() is true or false. the loop will then continue until either condition is met. Afterwards, just display the appropriate message based on the outcome. in pseudo-code terms:
//game loopdo{//this is where your game logic will go} while (DetermineVictory() == false)if (hasWon == true){//display victory screen}else if (hasWon == false){//display defeat screen}
