Use Photoshop 32bit with Lightroom
You want more RAM, therefore you have installed a 64bit OS.
In this case I am talking about Windows 7 64bit.
Adobe Photoshop comes in two versions, 32bit and 64bit. Both of them are being installed at once on your system. The 32bit version can utilize almost 2GB RAM for each session. The 64bit version can utilize almost all your RAM... But not all your necessary third party plugins have upgraded yet to 64bit.
Lightroom comes in two versions too. If you are on Windows 64bit, the Lightroom 64bit will be installed. The problem is that you still mainly use Photoshop 32bit (cause of the plugins) but Lightroom opens the Photoshop 64bit for further editing. It is a normal behaviour, the 64bit version is the default one, you are on a 64bit OS afterall.
I installed this seasons' Adobe updates and I applied the solution once more. I thought it may be useful to post it for anyone interested.
If your search your registry for the the Photoshop paths you will come to this key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows CurrentVersion\App Paths\Photoshop.exe
It has two values
Default value = "C:\Program Files\Adobe\Adobe Photoshop CS5 (64 Bit)\Photoshop.exe" Path = "C:\Program Files\Adobe\Adobe Photoshop CS5 (64 Bit)\
that you have to change into (the default Adobe Photoshop 32bit installation path)
Default value = "C:\Program Files (x86)\Adobe\Adobe Photoshop CS5\Photoshop.exe" Path = "C:\\Program Files (x86)\Adobe\Adobe Photoshop CS5\"
For your convenience I made a .reg file. Unzip it an double click it. It will make the changes for you.
Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:
322756 How to back up and restore the registry in Windows