Keyword order

Don't try to target different (and competitive) keywords on the same page. Instead, find (or build) the most relevant pages on your site for "x-box" and target your tags, content and links for x-box related keywords. Do the same for your "playstation 3" pages.

For your homepage--without knowing your strategy, or seeing your site--pick just one or two keywords to target and accept that unless you get a LOT of relevant links--using your keywords--then you'll likely not rank for both "x-box" and "playstation 3" with your homepage.

And as Rene said - don't worry about your keyword tags. Focus on your page TITLE attribute/tags and the content of the page itself. Oh, and links.

In the scheme of effective SEO strategies using your meta-data, the keywords tag plays a fairly "insigificant" role. Of more importance is your title tag, and less so, your description tag.

If you are ultimately trying to optimize your keywords tag, use the principle of the title tag and place your more important keywords first. But IMHO, focus your attentions elsewhere - especially if you are trying to get top rankings for competitive terms such as X-Box and Playstation 3.