I disagree. If you are looking for an entry-level computer for web-browsing and iTunes and word processing, you can't beat the prices of the manufacturers. Right now, we are in a good spot, where the hardware really is ahead of the software, so with a little bit of knowledge, you can get a computer that can easily run anything that 80% of users will throw at it.
The lowest base-model Dell is $329. It has a single-core processor, so I'd pass on that, but for $40, you can upgrade to a dual-core processor.
But if you keep looking, you can get an Inspiron 620ST for $450. This comes with an 18.5" monitor (the last Dell monitor I saw was pretty nice), 4Gb of RAM, and a dual-core processor.
The Dell Online Store: Build Your System
It would be difficult to build a PC at that price, with an operating system (Windows alone will set you back $100) and a monitor without using some bottom of the barrel parts.
I build my own PCs because I'm a gamer. As a gamer, I can save considerable money over Alienware and get only what I want, not what they want to charge me for. When my dad needs a new computer, I'm going to find him a Dell and move on. It's not worth my time to try to match the price.
I picked the same processor, a HDD that's almost as big, the first motherboard I would trust that has a DVI output (for video). Memory, PSU, case are all the ones I'd choose for a family member. First decent item I came across when sorting by price. Last two items are guestimates. Price: $573.
NewEgg prices:
OS: Win7 - $99
HDD: 750Gb Western Digital - $110
CPU: Intel G630 - $80
Motherboard: MSI H61M-E23 - $70
RAM: Crucial 4Gb DDR3-1333- $20
PSU: CoolerMaster Elite 460 - $30
Case: Rosewill R101-BK - $30
Monitor: Hanns-G HL193ABB - $90
DVD - $20
Keyboard / mouse - $20
We can debate Intel / AMD all day, but with OS / HDD / RAM / PSU / Case / Monitor / DVD / Keyboard / Mouse, we are at $425.
Can you maybe upgrade a few parts and get better bang for the buck? Maybe slide in an i3 processor for an extra $20? Sure. But for general consumer use, you can't beat the big manufacturers. You just have to find the right deal.