Flash for T1i - best cost x benefits considering 18-200 lens

Dear friends:

I'm new in the field.

Certainly there is "mininum recommended correspondence" between flashes capacity (power) and focal of the lens.

I read some advices for T1i, for general purpose photography, of 430EX or the equivalent Metz (Metz 48AF-C).

Are they "recommended" for lens like Canon 18-200 or Tamron 18-270?

Thank your for your advices.

Best regards,

Frank