Being cased and graded adds to their value (only if they receive a PSA 8, 9, or 10 grade, though), but if you don't do it through PSA, most collectors will pay less than they would for a PSA graded one because PSA seems to be the only widely accepted grading agency for Pokemon cards (even though from a technical, gem mint standpoint, other grading agencies have more stringent standards for a 10 grade and should therefore have more sought-after grade 10 cards).
If you're confident they're in near mint or gem mint condition, definitely get them graded. I recommend PSA simply because of the value the collecting community places on their grades, but if you're not comfortable dealing with PSA, then feel free to use whatever grading agency you are comfortable with.