I have two machines with XP x64 and Vista x64 both with 8GB and work a lot on files many times bigger, though its a struggle at times, I am not experiencing this kind of slow downs. I am wondering if you are also running out of hard drive space, as in there is no where for Windows to work its page-file (you may be using a dynamic PF, as oppose to a fixed size) I use several Page-Files, but all fixed spread over several hard drives, these are 50 GB each x 3 over 3 drives, non which are the OS disk.
I think that someone here, explained that working with tiff files is suppose to be faster, but when I do my pano's i use PSB (Photoshop Large Document), and I just opened a 2.3GB file from a separate hard drive from the OS, (this is important) and not including it prompted me for missing color profile it took between 1-2 min or at least that is how it felt, didn't time it.
I don't know how your computer is configured, but here are a few tips
Keep the the OS/applications on a separate disk, e.g. Disk 1
Scratch and page-files on a disk that is physically separate from the OS/APP disk, e.g. Disk 2
Keep your data files e.g. Images on a third disk. Ideally this should be at least be on a Mirrored Disk (eg. minimum of two disks (two of the same disks!)) also called RAID-1
Now, all these can be a derivatives of RAID, e.g. multiple disks. RAID-0 is for speed, which is ideal for both OS and for your Scratch disk(s), RAID-1 is for redundancy (Mirror), any RAIDs above this are a combination of these, RAID-5 is the compromise, Using 3 disks as a minimum, though I personally prefer RAID-10 (Mirror and Stripe) using 4 disks as a minimum.
One more thing to note, is do NOT use the cheapest disks for raids! Its like using a plastic lens on your new PhaseOne P-65+, only thing here is that one day your images won't be there if you don't get quality disks.
If you are opting to use cheaper disks eg. Green disks, then don't raid them. They are not made for this kind of applications. However, if you do have Green disks and you wish to get a bit more performance out of them, you can partition them, so that you have a small partition first, e.g. 15-25%. This is the outer edge of the disk and it therefor spins faster. You can partition a second partition and use this for your storage, this is "a poor man's performance improvement". However, its a cheap way to get a little better hard drive performance on a cheap disk.
I hope this can help you a bit, I know the hard drive configurations, can be a bit of a headache, as you add more drives you also need to add Hard drive controllers.
Makes you want to go back to film :-) (shot, drop off at lab, pause, pick up from lab, deliver to client -> ready for next job, Repeat loop)