Rather than limit your memory to 512MB it is probably better to just limit
vcache to 512MB which then allows your 1GB of RAM to be used when required by
applications.

For more details see:
MS KB 253912 - ""Out of Memory" Error Messages with Large Amounts of RAM
Installed" (http://support.microsoft.com?kbid=253912). The reason is that
allocating more than 512MB of RAM to vcache will exhaust all available upper
memory addresses and thus prevent them being used for other purposes.  This
problem is easily fixed as follows:
To limit vcache, open system.ini and add the lines
[VCache]
MaxFileCache=512000
Note that this will still mean that all  memory is available to applications
but is simply limiting the amount used as virtual cache memory.

If you really do want to limit your memory to 512MB, then this can be done
using MSConfig (Start, Run, enter MSConfig in the box and click OK), open the
General tab, click the Advanced button and you will find a box where you can
limit the amount of memory seen by the system.