One nifty digital dictionary for Arabic-English and English-Arabic is provided by Aratools, at http://www.aratools.com
You can use it for free online, or download a stand-alone application for Mac OSX. There is also an iOS app, which can run offline, for which you pay $15,-. You simply type the word as it is; no need to figure out the roots first. In fact, after you have typed your word, it will give the root letters, so this dictionary also comes in handy when you jut cannot think of the right root letters.
Its main advantage over competitors is that it is fully digital, i.e., you are not looking at scans of a hard copy dictionary. What it does really well is to supply suggestions as you type (online version). What is nice as well is that it gives vocalization. Less good is that it typically only gives one meaning, which tends to be derived from modern usage. The source is actually not given, so you have no way of knowing if the meaning it gives to you is applicable to the context you need it for. Use with caution.
The website seems to be optimized for using on a smart phone, making it an ideal buddy for when you are away from your trusted hard copy dictionary. If you do not have stable Internet access, you may want to invest in the app (iPhone/iPad only).