How uniform are quanta's motherboards?
 
I'm curious from an upgrade perspective. When I bought my HP/Compaq I knew at the time i would be stuck with it's limitations and accepted that fact because it was a great buy at $500. But as a budding desktop homebuilder, I've started wondering about the feasibility of upgrading internal components since at least half of them are made by Quanta for different companies (Dell, Sony, Lonovo, HP/Compaq).  
 
I know I can always upgrade the processor, but my big question is the graphics chipset. Since laptops use integrated gpu's, the graphics are limited to the particular MoBo.
 
I don't think it would be much of a problem if I stuck to the same generation of case. For Example, I have:
 
Compaq Presario  V6305NR  w/ AMD Turion 64 X2 TL50 1.6Ghz w/ Nvidia GeForce Go 6150
 
and the top line V6000 series seems to be:
 
Presario V6700TX  w/ Intel Core 2 Duo  T7250 2.00 GHz w/ Intel X3100 Graphics
(although there seems to be a V6850 somewhere but I can't find documentation)
 
Now the Core 2 is obviously faster, about 30%, than the AMD 64 X2 TL50, BUT the Intel X3100 is only incrementally faster than the GeForce go 6150. So, as graphics go there's really no point.
 
But on the HP side, they offer the dv6700t with a VERY similar but not quite identical case that offers:
Intel Core 2 Duo 9300 2.5Ghz and  Nvidia GeForce 8400M GS
 
That's at least a 60% faster CPU and a much faster (though still limited) GPU.
 
So in theory it should be VERY feasible to upgrade for the same case. But would it be possible to go even higher performance with other quanta components, or do you think case limitations from company to company, model line to model line would make it prohibitive if not impossible. AND would you think the performance limitations set by the    company should be observed in the interest of cooling and heat buildup in such a small case?
 
I also know that changing the internals violates my "agreement" with HP and will probably render my OS useless, but I'm really talking hypothetical at this point (and I hate Vista anyway). Like I said, it's hypothetical right now, but if it seems possible I'd like to start working on the concept before the current generation disappears while documentation is still available.  
 
What do you think? Those with experience opening various brands have a take? Think this would be better suited to homebuilt?