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.