First of all, you have to use a multilanguage plugin to have the frontend in Hebrew & the backend in English. There's a free plugin called polylang that'll do it: http://wordpress.org/extend/plugins/polylang/ but the paid plugin @ wpml.org is much more comprehensive. It'll also help you to easily translate text strings in the template & plugins easily. The primary function of both is to have multiple languages in the frontend though.
I've managed to convert many templates' CSS to RTL successfully using the following plugin: http://wordpress.org/extend/plugins/rtler/
That plugin saves you hours as it mirrors everything for you. Just make sure that you apply it to all the css files used for your template. On Kriesi's Eunoia template - I had to apply it to base.css & layout.css and then combine the results in a new rtl.css that is placed in the template root.
You'll still have to fine-tune some stuff - but not too much - it literally saves you hours.
The only problem that you'll have is with framework specific stuff (with Kriesi's templates that's the avia framework stuff). Nick is very kindly helping me out (thanks a lot) with a specific issue regarding the portfolio framework.
I suggest you checkout the above-mentioned plugins. I'm not an expert developer by a long-shot - but I've managed to setup numerous Arabic & English sites using non-RTL enabled templates by using the above plugins & some help from template developers for their framework specific stuff.