Well worldwide takings for X-Men: First Class stand at (according to boxofficemojo)
Domestic US$119.925m (42.5%)
Foriegn US$162.100m (57.5%)
Of course other movies are coming out now which might have a knock on effect, so my guess for the final takings will be in the US$300m to US$320m
Which I think would be good enough for their to be a sequel.