I'm guessing your impreza .psd file is quite a bit larger than the other files you're trying to open???
First of all try increasing the memory usage allocated to PS. With PS open hit CTRL+K to bring up the preferences, then find the page called
memory & image cache, then check the 'max ram used by Photoshop' setting.. ideally on most system about 60% is what u need. It all depends on how much ram you want to let PS hog. Obviously more makes PS better, but it means the other programs you got running at the same time (including your OS) may suffer... After editing preferences you'll have to restart PS for the changes to come into effect.
Scratch disks are the parts of your HDD that photoshop uses as temporary storage space, basically its uses scratchdisks when it runs out of RAM to use.. so thats why i told u to bump that up firstly.. If that still doesn't help out then it may be a sign you're running out of diskspace or you need to defrag your HDD.. Some more info on scratch disks on the adobe site:
http://kb2.adobe.com/cps/322/322829.html