One thing that you should be aware of is that some search engines (namely Google) do have some penalities if you go crazy with keywords.
You should be careful about doing too much keyword stuffing. In particular, avoid the obvious stuff that Google can detect easily with it's algorithms (for example: stuffing keyword meta tags).
On the other hand, keyword stuffing is highly effective for Yahoo. So it is a balance.
Let me know if you want further thoughts. I'm a professional SEO consultant.
Ben
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crispyseo@gmail.com for your free SEO review!