As a number:
* Three hundred and fifty
* Three fifty
As currency:
* Three hundred and fifty dollars (US, Canada, Australia)
* Three hundred and fifty pounds (UK)
* Three hundred and fifty euros (Europe)
As a decimal:
* Three hundred and fifty point zero zero
* Three fifty point zero zero
As a fraction:
* Three hundred and fifty over one
The best way to spell 350.00 depends on what you are trying to communicate. If you are writing a check, you would use the currency form. If you are writing a report, you might use the number form.