Ok - probably hard to answer but I have DNN on a shared web hosting provider - on decent Xeon servers but shared....

I know its nearly impossible to answer since every farm and setup is different but whatsimultaneous users should I expect to be able to handle...(on an average shared web hoster)

Would it handle 150-200 users pretty ok?

Also can anyone recommend a stress testing tool they have used with DNN?

I'm also in need of such a tool.

I found one at MS: http://www.microsoft.com/technet/archive/itsolutions/intranet/downloads/webstres.mspx?mfr=true

but it does not work on vista  (nor xp)

It looked like a great tool, maybe this link motivates you to search further.  If i find an up-to-date version of this i'll post it here for sure.

By the way, you can easely find this tool if you want to pay for it:
ex.: http://www.radview.com/home.aspx

You find them on the right if you search on google:
http://www.google.be/search?hl=nl&q=microsoft+web+stress+test+tool&btnG=Google+zoeken&meta=