I got a similar Acer 12 months ago and it's been fine. I saw the 1640 you mention in Comet for £259 (without a monitor) and I paid £400 a year ago. I thought it was extremely good value.
For a 14 year old, I would suss out the type of games he will be playing and I might suggest the 9300 graphics card may be adequate, but you might still need to make further enquiries on this - I tried Colin McRaes DIRT and it run like a wounded snail on my system which is :
Athlon 64 x2 Dual Core 2.6Ghz 2GB RAM Vista HP NVidia GForce 7300SE
If he is only interested in games why not just get him a games machine. xbox etc. Sorry not a games person so don't know the various machines. Proably cost less & be built especially for the job. & apparently some now come with internet access so that games can be played online which I suspect he may want to do anyway.
Some say Acer are rubbish but 1 know 2 friends who would not buy a different make! Dell are good, but sometimes under spec for the price. I bought Toshiba as my last laptop lasted 10 years of daily being used!!
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