Thanks for your input guys. I will definitely forget the Z option and carry on looking for a non-hyphenated alternative. Since reading your replies and taking on board your advice, I have found a couple of possibile alternatives and I am even considering a domain without the keyword in at all.

Sadly, EGOL, buying www.keywordguides.com is probably not an option. The budget for this project is tiny... I guess I am just going to have to be a bit more creative

Possibly seems a strange question, but let me clarify...

I have a new site in mind and all the domain names I was considering for it have been taken (I want a .com or a .net if at all possible). However, I can get the domain with a z at the end rather than an s

Example: www.keyword-guides.com is taken, but www.keyword-guidez.com is available.

Am I completely wrong in thinking that it will still match well for anyone searching Keyword Guide, and should match fairly well (even though it is a partial match) for people searching Keyword Guides.

As the keyword is the most relevant bit of the domain, and as the first word on the domain is given the most weight, will having Z instead of S at the end make any difference at all?

Personally, I don't really like the Z option, but if it would have no (or little) impact on my SEO efforts, I could live with it.