| Built a few.
My advice is to buy the technology that is about six to twelve months old (unless you really want the latest at a big premium) ie P4 2.8 or Athlon 3000xp £100 or less and more than quick enough for everything in use today. Get a mainboard with 8xAGP, a 400mhz Front side bus and at least 5 PCI slots form a reputable manufacturer (Asus or Gigabyte are good) for around £50 and you can't really go wrong. A 7200rpm HDD of whatever size you want or can afford.
The graphics card really depends on whether you intend to play the latest games. You can get some pretty good 8x ones with 128mb ddr for around £70 (nvidia or ATI chipsets).
Build the one you want online in the shopping basket using ebuyer, dabs, aria or Komplett and see who is the cheapest. | 
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