During Initial versions(RTM) of SQL 2008 and R2 I have observed multiple posts related to the error “There is insufficient system memory in resource pool ‘internal’ to run this query”.
Mostly the error comes up if a Full Text Query is ran which uses compound words,however the same error comes up on different situations too and there was a BUG which was opened for this case too
The good news is that SQL Server Product team have fixed this issue and if you are facing this error in your systems(RTM) right now,then you can go ahead and deploy the latest service pack for SQL Server 2008 and R2.
SP3 for SQL Server 2008 –
http://www.microsoft.com/download/en/details.aspx?id=27594
SP1 for SQL Server 2008 R2 –
http://www.microsoft.com/download/en/details.aspx?id=26727
< Note Added on 8/2/2012 >
Incase you are having severe memory pressure,then also you can notice this error message and in that case it has nothing to do with service packs.You will have to closely verify and set MAX,MIN memory settings along with LPIM.
</Note>
SQLSailor 
Wow. What a great post. During a deployment this problem suddenly appeared. SQL is patched and running like a charm now. Thank you!
Glad that you liked it Steve,Thanks for your comment.