After mail-in rebates, you've got $350 to play with for case and storage.
You think that's comparable? 2.4GHz vs 2.93GHz.
PC6400 vs PC8500.
SLI capable motherboard vs non-SLI motherboard.
It's also missing:
Storage
Optical drives
Case
If I wanted to build a humble computer such as that, I would've. Unfortunately, it's not nearly as fast nor as expandable as the system I've gotten now. You may think it's overkill, but for what I use it for, it's not.
I've yet to see a test where the difference between factory OC and Ultra is more than 2-3%.
Well, duh, since the architecture is the same. The key point of the Ultra is it can clock faster. Please learn about the A3 stepping and its advantages in the G80.
Even so, almost every example you posted is irrelevant in real world performance - you're paying bragging rights, and little else. Truthfully, any 8800GTX will slaughter basically any DX9 out there now, and all of them are mediocre at best on DX10.
This is nonsense on many levels.
How can something be "mediocre at best" when there is nothing better?
Additionally, I suspect you know next to nothing about how these modern GPUs work. Here's a little hint for you: there's nothing fixed-function DX9 about them. They are massive MIMD engines with lots of small FPUs. There is no different to the hardware if it's DX9 or DX10 -- the difference is to the programmer.
FWIW, I'm a hobbyist games programmer and I'm working on porting Ogre3D to DX10. I'm well aware of how it works, are you?