I currently have basics

evga gtx480 1gig
i5 2500 (locked)
8 gigs ram

New game is Guild Wars 2. Currently I have been able to play pretty well with setting on high however I have an older 20 inch lcd monitor so my res is lower. I have it maxed but only goes so high (forget what it is off hand)

I would like to maybe upgrade the monitor to a 24 or 27 inch screen and maybe bump up the card. However my budget isn't very high. 500 bucks max. I guess I"m wondering since my current card seems to play pretty well is if I did upgrade my monitor and played at a much higher res would I loose a lot of FPS? Is there a card worth upgrading from a gtx480 that can be had for 200 bucks?