There is some evidence that shows that machines with 8GB RAM respond, and wake from hibernation etc faster than machines with 4GB. Then again, it's from RAM suppliers (Corsair and i forget the other). Allthough real world users do report it's true to an extent.
One thing i can vouch for, and i definetley notice, which is to be expected really, is that with 8GB RAM over 4GB, the machine responds quicker, when you have multiple applications left open.
A lot of people got into the habit over the years (particurlarly before 4GB was seen as the baseline) of closing an application when they weren't using it. Of course back then, the swap file located on the mechanical hard drive became inundated, but even when 2GB machines became the norm people still close applications.
These days (and as has been pointed out it depends what applications you use) the number
of applications you can now leave running, without even bothering to close them, has dramatically increased, to the point where you can actually disable the swap file, or (as some apps still need to know its there even if they dont actually use it) at the very least make it tiny, as the extra RAM will pick up the slack. So, long gone are the days of staring at the HD activity light waiting for the page file to catch up.
After my own testing, I found I do notice a (what I would personally deem) significant difference when using 8GB over 4GB and i've now go into the habit of leaving pretty much all apps open, whereas before i used to close them. I'm a home user so the apps i'm talking about are the more common ones.
On that basis, i'd personally say yes, its worth going up to 8GB.
You have another option (assuming you dont already have one), which would give you a far more noticable affect on performance: an SSD drive which you will install the OS on.
So, even in view of all the above, and again assuming your machine doesnt already have an SSD, i think overall you would be better putting that £40 toward the cost of an SSD. Smaller ones are fairly cheap nowadays, you could still keep your existing mechanical hard drive[s] for storage, back up, extra programs etc. Of course as yours is an OEM pre built PC, you may be reluctant to reinstall the OS, but you would notice the affects of that SSD, above the transition between 4GB and 8GB RAM (assuming you dont use a lot of intensive apps at once) in my opinion. The mechanical hard drive is still usually the bottleneck in most of todays PC's.
Of course if money is no object, get both
Edited by RiffRaff (Mon 30-Apr-12 13:40:31)