I'm not trying to persuade you otherwise, however building your own or asking someone else to stick it together for you has major benefits.
For example, my family's HP has an onboard video chipset and onboard sound chipset. Also the space is crammed with very little space to work. This makes cleaning, upgrading, or tuning extremely difficult and in turn wears down the hardware. Now, you could build one (or once again, ask someone to stick it together for you) yourself for a cheaper price and at the same time you get hardware that is more durable, and a computer that actually works better.
The only downside of building your own, no warranty. Which is (in my opinion) the only thing which makes buying a pre-built computer worth it.
What you didn't mention about warranties, is that all the parts in the system come with manufactures warranties ie most memory Lifetime, most hard drives 3 years, motherboards 1 to 3 years, DVD/Cd's 1 to 3 years. Also all systems come with an on-board video chipset I think what your meaning is on-board video meaning it shares the video with the system memory. So yes there is a warranty with systems build outside of a store! But what the stores call sucker "up-sells" is the extended warranty, they sell you an "extended warranty" then they send in the parts to the manufacture for a free replacement... no cost to the store.
I do agree with what your saying as far as the parts, you get garbage or retired components for the most part from stores like Circuit City, Best Buy etc. The reason for systems being sold at 500 or 700 or 999 is the parts are old old old, 18 month old technology or inferior parts like DDR2 that runs at the slowest system buses, you will not find the mid level boards or graphics cards in those systems, only entry level with today's technology at best. I posted a system for 1500.00 that will run circles around any dell, gateway, or other store bought machine costing 1 1/2 to 2 times as much.
Lots guys here play this game and if they get a decent frame rate they are happy, but what if they decide to play something else, the monies they spent on the machine for AHII is wasted since the machine was not build for "Gaming" from the start. Check out a program called 3DMARK06
http://www.futuremark.com/benchmarks/3dmark06/introduction/ its what is used to benchmark machines. If your not in the 11000 mark or above then you have a system that is slow and your machine will stall with most of the newer games coming out. DirectX 10 is out and the gaming companies are starting to make use of this. If you don't build around DirectX 10 you will be out building or upgrading again when and if AHII takes that route and will eventually will have to.
As for Vista, yea its a big system resource hog, but again build the right system from the start with the proper components and no problems. XP is no longer being made or supported so the service packs are no longer going to be available. Soon XP new in the box will go out of sight. I used to pay 109.00 for XP PRO now its at 141.00. I agree with Skuzzy that Vista is an IT guys dream, very little to have to do with security but it is a system resource hog, back to build right from the start no problems.
Bottom line is this, the original poster here stated in another post stating he has 1500.00 to spend on a system, I posted in his thread a 1500.00 system that will score in the 13000 to 15000 range on 3DMARK06 with little or no tweeking for him to do, if he were to call Dell or Gateway for a "Custom" build of the components I listed he would spend 2400.00 plus but he would get that warranty from Dell or XYZ company.