data tables and data sets - single items

Hey,
A few dumb questions, but something that will help me out alot.

how do i get just a single value out of a data table (or dataset)?
(not looping through the data table..)

say - if i wanted the item from the fourth row  in the fifth column?

also, if i wanted all the values in the 5 fifth row of the datatable,
or if i wanted alll the values in the sixth column for all the rows?

AND...
one more...

i have loaded values from about 6 tables from a database into datatables, each database table corresponding to each data table
Now... i need to do some queries - like  - Selecting different fields from diffenent rows and columns according to results from another query - they are nothing to compicated at all, just getting generally one or two values from one table matching/checking it to another and then inserting somewhere else..

Should / can i do this with data tables - if i load data from a database (as i have mentioned above) can i then do rather select statements to grab info from them and then add to another datatable to update the database after all queries have been done...

in other words, is it possible to use data tables and close the connection to the database and do fitering, selecting and updating (and in some cases complex select statements) from/ to the data tables, then reinsert...
or do i have to use SQL select statements keeping the database connection open?


Please help me, i am in urgent need..
many thanks,
Bill