you are most likely to play on low to medium at thar price range.
you can grab a 700 series nvidia gpu that will come with watchdogs for free.
i whould recomend a gtx 760 for medium to high.
for more information visit the link below.you will need to guy a gpu from the places nvidia has on their site to get watchdogs free. http://www.geforce.co.uk/games-applications/pc-games/watch-dogs/bundle