Dell has released 3 models in the new Axim X51 family, an evolution from the X50 lineup. In terms of specifications, you will not find that much is different. The X51v is almost identical to the X50 except for 256MB of ROM with 190MB of it available to the user and Windows Mobile 5.0. The QVGA X50 models come with either a 416MHz or 520MHz processor and come with 128MB of ROM. Otherwise, everythings is the same, down to the way it looks. Given the nearly identical specs, you might want to consider trying to pick up an X50 (with free Windows Mobile 5.0 upgrade CD) and save yourself a little money.
The new lineup of X51 Axims is available for the same price that Dell was selling the X50s earlier. For example, the X51v starts at $499 USD ($599 CAD).