capacity of tha database to my ears means soemthign differnet to the querstions you are asking
theres various limitations on any server database
theres hardware realted things like the physical sapce available, the physical memory available, the speed tof the netwrok coinnection between the db and the webservers and the outside world
theres software or settinggs or design which affect performance which affects throughgput which in effect affect capcity
...poor design means theres more db activity to retrieve the desired data. that poor design may be because of poor indexing strategy (too many to few, badly designed). poor design may be because of poorly designed tables and columns, examples of this could be a non normalised db, using the wrong datatype eg using char/varchar for dates or numbers. poor query / indexing can be spotted by using the correct analysis tools
performance may be affected by the way in which rows are inserted and or how oftent he indexes are rebuilt, most of that should be covered by the DBA's maintianing the db
theres also consoderations of how the client application(s) access the data. ferinstance how long client programs seek to hold connections open, what tesopurces they consume, the amount of data they pump up and down the network. some of that is affected b y how good the client developers are in things liek SQL, do they use SQL or do they exploit SQL.
as to your other questions
that all depends on the servers. you can run an instance of a MySQL server on an 1.6Ghz PC quite happily, however that will not support as many users as a multi core high end processor.
I have a study and my teacher noted this
'servers - number of clients that manage; calculate number of transactions per day
Servers to be used (how many transactions can each server handle at
a time to make sure the quality of servers isn't compromised)'
So my questions are: How do i determine the number of clients that server can manage? If we consider the records of each client, how large would it be? Why am i having difficulty in answering this? Well, what i know is that each record would be an entry to a table. So it won't be saved as an individual record. right?
generally speaking more processor power, more memory faster network connections equates to more users, more transactions and so on. however usually the best performance gain is from properly designed tables, columns and indexes. its not as critical as it used to be but its still important that the basic design is right, the views are right and so on. after that it becomes scalable where adding or improving hardware resources increases the number of concurrent users and or response time
I'd rather be riding on the Tiger 800 or the Norton