Apples and oranges.
The Dimension 8300 has a 250 W power supply. Dell doesn't offer the Iiyama monitor. I'm not up on memory these days, but you spec'ed PC3200 DDR RAM while the 8300 uses 400 MHz dual channel DDR - I don't know if they're equivalent. Dell typically skimps on memory speed.
I get about $3800(2400 pounds) for a 8300 per your specs (but the 200 GB drive is Ultra ATA, not SATA, the speakers are different, etc.) from Dell US.
It often does cost a little more to built-it yourself. But it's important to compare apples to apples - i.e. you can probably get better parts for the same price as the big PC houses.
My $0.02.
Cheers,
Scott.
