High Sqlservr.exe Memory Usage

The counter "Target Server Memory", 8428480 KB is how much SQL Server would like to use You only have 7978472KB allocated to the instance ("Total Server Memory")

I suggest you add more memory or move to x64 if you haven't already

However, if you PAE and AWE enabled you must set max server memory on 32 bit. Otherwise it grabs the lot except 256 MB.

Finally, SQL Server is designed to use memory. Another SF question about it

I'm assuming you've configured SQL to use AWE. If so, then it's normal for SQL to allocate all the memory and release memory to other processes as needed. This article explains how AWE and allocated memory work in SQL Server.

http://technet.microsoft.com/en-us/library/ms190673.aspx