optimistic concurrency with timestamp and typed dataset in asp.net

Question

i'm creating a forum system for my web site with the use of c# and asp.net and for data access i'm using a typed dataset and for UI i'm using mvp pattern. in my database i have stored procedures which i have been added to my dataset. the problem was that the dataset didn't allow me to select the "use optimistic concurrency" check box so i had to implement the optimistic concurrency myself. i've been added a timestamp column to my tables now i have a problem with saving this value for each user! so what state management approach is safe and less costly to be used for saving the timestamp value??

i thought maybe it's best to use session for storing the value but if the user's count goes up i would have a problem with memory resources however i can set the session to use sql server instead of memory but i have no idea how much slower that would be so i can't decide about it.

any help is appreciated guys!

Edit1: yes. i want to track the last modification of row to see if it matches with the one that user has. but i don't know where to save the user's timestamp value.