Photoshop :: Scratch Full And Updating System

I'm running into scratch full problems often with my 5 year old system and am not certain I've got it set up properly to know how soon or with what I should update/replace it.
 
When resizing or sharpening my biggest file for printing, I'll often run into "scratch is full errors", but when I look at the temp file in the scratch disc when this happens, it's only a 4-6gb file. Does this sound right? The drive has 135 gb free space and is a 10,000 rpm 175gb raptor.
 
I currently use XP64 on a Q9450 on a P35 ASUS mb with  8gb ram (maxed), 256mb ATI graphics , a 75gb raptor for OS/programs, a 150gb raptor for scratch, and data is on two 1TB drives. I have an update to WIN7 pro disc which I will likely do on a 240gb SSD (replacing the 75gb raptor), but am trying to figure if I've got this optimally set up before I start making changes to sort out where to make changes first as I don't have a huge budget currently. I know I'll have to update the whole system very soon, but am trying to decide a route to take.
 
The file I'm working with (which is my largest so far) is a stitched panorama ~650mb without layers. This regularly sells as a 12x48" print. I'm already looking a making much larger stitched panoramas than this. In fact, I've got the files, just don't have things set up for them yet. (D800 camera needs updated lightroom, capture NX, etc, but want to figure out how to update this system even minorly to keep using for administrative stuff once it's replaced)
 
I'm looking at putting in some parts that could then be transferred to a new chassis when the time/budget comes. In other words, building it piece by piece.
 
240gb intel 520 ssd os/programs
480gb intel 520 ssd scratch
 
Basically with this, I'd just be moving the 480 ssd to a new system and keep the old one going with the 240 SSD (though I wonder if a $99 Crucial V4 (ie sata ii connection) 128gb ssd might be a better investment for something that will stay in this computer/motherboard).This seems overly complicated now that I read it over. Basically it comes down to these two questions:
 
-Do I not have my current scratch set up properly? Seems weird for it to be full with only a 4gb file.

-would a crucial V4 be a better ssd to speed this system up (and keep going as a second computer later on) then the 240gb with sata iii connections?