The specification should be a balance between affordability and the intended use.
Large binary files - videos, music, graphics of any kind - all use a lot of storage memory and need a lot of processing power.
Even the compressed formats of mp3, xvid and divx take up a lot of space when used to store files numbered in the thousands.
For example, a 4.5Gb DVD file compresses to around .7Gb judging from what's available on movie download sites.
That's a factor of just under 7 - which is not a lot if you abaolutely have to have every teen movie ever made
Office applications - such as outlook, access, word, excel, to some degree powerpoint - generally do not consume as much resources.
Browsing with multiple windows takes a lot of current memory [RAM: as opposed to storage memory = Hard Rrive size].
Working with large image files or CAD files can consume resources even if the files aren't too big themselves - multiple undo options and temp files will eat up resources.
Then you can have incompatibilities and memory leaks.
For some reason Winrar used to hog resources on my WinXP system.
Changing the default rar and zip file opener to z-zip sorted that out, allowing me to browse image files within stored archives without having to uncompress them.
For 5th Year Secondary student - aim for a 2.2Ghz CPU with 250Gb Hard Drive and if its Windows Vista or 7, as much RAM as you can get - 2Gb is the minimum I would recommend and preferably 3 or 4Gb.
The screen should have high refresh rate and contrast ratio and be a minimum of 15" for ease of use, with 14" for max portability - I hate these tiny little 10" screens these web-books have - but that's just my personal opinion as I work a lot with CAD and image files.
If the student does need graphical work done, a graphics card is very useful, but can hugely increase the price for some reason.
The usual workaround for other manufacturers is to run the graphics through the motherboard and RAM and "dedicate" some RAM for this - often seen as being the "cheap seats" in PC terms, but for that very reason of cost acceptable to most except hardcor gamers and photography or graphics professionals.
Most makes are good these days with plenty of Acers, Toshibas, and even Dells in the shops - but don't impulse buy or be too led by a salesman.
Go online and type in the model you're interest in a google search engine with "review" or "comparison" and see if you can get an online review in one of the better PC mags or tech forums.
Some of these may be too technical for the layperson but you should still be able to see from the discussion if there are any major problems.
For example, there is no point having all the bells and whistles if the unit runs out of battery in 6 hours of an 7 hour school day...
So you need to find out from people who actually have used the model whether or not it lives up to the manufacturers claims of battery life - that's where the in depth reviews come in.
Sometimes the PC/Laptop Mags in the shops offer good reviews - but remember, they get their advertising in part from people who sell laptops.
Finally unless Microsoft and Intel Products are essential there are products running varients of Linux on AMD chips out there now.
These seem to be quite affordable products with a very reliable operating system.
Balancing desirable features with necessary ones - that's the key.
FWIW
ONQ.