Largely depends on how much you can afford to spend. You can find chairs that cost a fortune (£1000+) or a little bit less.
Viking-Direct click here is a good place to start looking.
Arms are a personal preference though I have a feeling you aren't supposed to have them if you do lots of typing.
I've got a nice high back leather one that does the job for me. No doubt slowly killing my back but has comfort for now!