intel and AMD need different motherboards so if yu do go the build it yourself route be aware of the different m/bs out there.
If you have a decent HD already and a decent video card,Monitor 17"+ and say DVD i would upgrade yourself,
however if you dont have all these i would go for a generic (shop built) computer as often you will get it for about the same cost and it will have extras like printer etc thrown in (+the garentee) although each component you buy has usually 3 yr warrenty.
I had a P2 350 with 64mb and voodoo3 and i bought a Abit Ka7a raid mobo a 900Mhz tbird AMD and a geforce 2 MX.
But this meant i had to buy a new towercase £30, new memory because it now supported 133mhz.
It all adds up and sometimes for that little extra you can get decent systems.
My advice? you have an old computer with med-small hardrive with a 15" monitor and you want to upgrade?
BUY new branded system (not necessarily DELL,HP etc,these guys are a ripoff! find a decent local computer store and buy theirs)

Got a big hard drive (17Gig+) good monitor(17-19") lots of other components(dvd,vid card) and just need a boost in speed?
Buy a m/b processor and towercase and gut your old one to make a new system.
My jump from 350 to 900 was fantastic and cost me arounf £400 with the vid card and other bits.
theres no way i could get an equivalent system for that price from Dell etc.
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Hazed
9./JG54