Hellsing the OVA Series is a brilliant piece of work. It follows the manga much better than the original anime series, although the soundtrack to the original was brilliant, so is the OVA's.
Alucard is more deep in character than the last time and so is Anderson (even more so,) and many scenes that didn't make the anime appear in the OVA.
The graphics outdo the original series (and most other animes for that matter,) as they utilise cgi in important scenes. The same voice actors from the original show are here, along with a few new ones.