[SQLObject] Efficiently updating rows?

I've got 100,000 users running my app which stores their 
data locally. I'm now releasing cell phone apps and need 
to sync the data through a web server.

Each user would have on average 10,000 rows of

item:
   string item_name
   int  days_since_last_update
   int  item_value

What will happen about 50 times a day is the server will
receive a list of 5 items to update their values ( if current 
day is greater than that row's days_since_last_update+2 ) ...

So given item names: A, B, C, D, E whats the most 
efficient way to get those 5 items and then update their
 values if the current_day is greater than that items 
days_since_last_update? 

Also each row is actually about 200 bytes. Thats 2mb 
per user or 200gb for the database? Do I need to be 
concerned about performance of my MySQL database? 
The only other operation would be returning a query of 
all the rows for that user with days_since_last_update 
greater than some_value..