I would suggest you contact Tableau Sales if you're interested in purchasing Tableau and they can talk through your specific requirements.
If you're an existing customer, contact your account manager.
Often the width of the data set (how many columns) will matter more than the height (how many rows), but anyway...
The "performance" of Tableau Server is not trivially predicted by formula alone. Aside from issues like how many concurrent users one might expect, whether or not any Worker hosts are clustered with the Server, and so on...
Performance is also greatly affected by the construction of the Workbooks published upon it, e.g. whether or not the Data is Extracted, whether the Workbook contains a great many Worksheets as Tabs, whether or not a large number of Quick Filters are employed, and so on.
There are many whitepapers and blog posts available on this subject; just search for "Tableau workbook performance."
Now for the good news: (1) There are tools like Performance Recorder and now TabJolt available from Tableau to help with optimizing and (2) 9.0 will generally perform faster than did 8.x , like-for like.
The best way for you to really know how it's going to go would be to obtain a Trial license key for Tableau Server, install it on your host, and try it out. There is no substitute for the real thing.
Extremely well put Ken!
Thanks Ken.. for such a brief response