I've only got SQL Server 2005 Express, not 2008, but from SQL Server Management Studio Express, if I right-click on the root node in the tree (the server node) and select Properties, there's a "Memory" page with both minimum and maximum amounts of memory available to be set.
From the docs for these options:
Minimum server memory (in MB)
Specifies that SQL Server should start
with at least the minimum amount of
allocated memory and not release
memory below this value. Set this
value based on the size and activity
of your instance of SQL Server. Always
set the option to a reasonable value
to ensure that the operating system
does not request too much memory from
SQL Server and inhibit Windows
performance.
Maximum server memory (in MB)
Specifies the maximum amount of memory
SQL Server can allocate when it starts
and while it runs. This configuration
option can be set to a specific value
if you know there are multiple
applications running at the same time
as SQL Server and you want to
guarantee that these applications have
sufficient memory to run. If these
other applications, such as Web or
e-mail servers, request memory only as
needed, then do not set the option,
because SQL Server will release memory
to them as needed. However,
applications often use whatever memory
is available when they start and do
not request more if needed. If an
application that behaves in this
manner runs on the same computer at
the same time as SQL Server, set the
option to a value that guarantees that
the memory required by the application
is not allocated by SQL Server.
I'd be surprised if these options weren't in 2008, but you could always just install it and try.