How many list items could the search box access and still maintain the extremely speedy responses to keystrokes:
http://ajax.zervaas.com.au/examples/...ggestCloneJax/

I need an educated answer. I'm thinking about implementing it on a site with several thousand names, and I might hire someone from the forum eventually.

That's hard to say because of the fact that your application doesn't access data on its own server. You have to factor in things such as the server it's accessing (in this case, one of big G's), any issues it may have in terms of reaching the server (although probably minimal in this case), any issues that may arise between your server and big G's (most likely scenario), etc.

I'm not really sure you can make that kind of a guess.