Office Ergo has some tips for selecting computer seating. Key elements to look for in a new chair:

1) Adjustable Seat Height

2) Seat depth Adjustability

3) Backrest height and angle Adjustability

4) Chair Recline

5) Seat pan angle Adjustability

6) Lumbar support and Adjustability

I have had problems with arm rests in the past.  Typing with arms on an arm rest lead to problems.  I ditched my arms.

You may need to goto a computer chair or ergonomics speciality store and not an office product super store to find your optimal chair.  Try a chair before you buy.