what do you want to do with your pc?...
for game or others??.....
if you play game for the most of time ...
then go for the MX400.....
if not....
go for the 550..............

there's no need to know the speed of VGA card...
you have to know your needs first...
but from your post...
i can't see any your needs..
so there's no information i can gather....
the sugguestion i just gave you...
may not be the best answer for you..
but this is the best i can give you

so ....
next time if you are going to ask a question like this...
pls tell us more.....