Good luck, you'll need it!
Have you any idea how long it takes to design a motherboard? For such a system as you proposed, from scratch would take upto 3,000 hours excluding any programmable logic.
The Emotion engine cost Sony, 500 Milllion US Dollars and 2 years to develop. Think you can still design your own chip?
AGP will be obsolete by the end of this year. Take a look at PCI Express, that is the replacement bus for graphics stuff. If you are serious, get some NDAs with IBM and look at the PPC970.
Unless you are going to sell 10,000 units a week, Nvidia or ATI will not talk to you.
Designing the system is maybe 30-40% of the work, the rest is the verification of the system, bear this in mind.
I'm not trying to completely dismiss your idea, just provide an insight into the amount of time and effort it takes to develop cutting edge electronics. I develop cutting edge computer systems for a living, it costs millions to develop such a system, it puts hairs on your chest, as you pull them out of your head in frustration at times.
Still you've got to dream.
Ian