Only using it for office work and for some relatively old MMOs (wow, lotro), so don’t need anything high end, but still looking for as big of a performace boost as possible with this system and PSU (250W). Directx 11 support would be welcome if possible. Budget is up to 100-150$ roughly.
Best bet would be an ATI 6570. Go with any manufacturer that offers the low pro bracket. That chip should run WOW no problem and it has a low power draw so it will work well with the small PSU. It does not put out a lot of heat so it should be ok in that small case. Newegg and Tiger have them in the $70 to $80 range.