The only real difference between the "Flash" OC and the Coolbits OC is your ability to get granular. NiBiTor will let you adjust area's of your card that 99.9% of us DO NOT need to be touching. Most folks just use it so they can adjust the 7800's voltage level's in accordance with the OC'd frequency (In case you did not know, when you up the frequency on a microprocessor of any kind, you will reach a point where the voltage needs to be raised in accordance to the clock frequency. This is so your chip will run stable when so much is being asked of it. It's kind of like pushing your heart in a marathon without enough oxygen!).
Coolbits works fine and unless you're a real pro, YOU DO NOT need NiBiTor. Flashing a BIOS is dangerous and I don't feel it's worth risking a $600+ GPU.
As far as better results from the M570A versus the D900t, you must remember "Better" is highly subjective. It really all comes down to the CPU's, the M570A uses the Pentium M, this is the most efficient production CPU ever designed in terms of pure efficiency. It uses a low clock frequency at a low voltage and processes data incredibly rapidly because of a huge 2Mb L2 Cache and a beautiful short pipeline architecture. So say you have a Pentium M w/ 2Mb L2 Cache (This is a repository on the actual CPU dye for storing data on the chip and accessing it as needed versus having to access mainboard memory all of the time with a smaller L2 cache) at 2.13 GHz, this CPU has the same effective processing performance for a single task as a Pentium 4 HT running at 3.6 GHz with a 1Mb L2 Cache. So as you can see, it's two ways of doing the same thing. Now there are variables, the P4 will Multi-task better, it makes full usage of WIN XP's HT code and the P4 nearly becomes two processors in one as it will process two CPU instruction sets at once. This is a Windows XP thing unfortunately, so the Multitasking will be better on the P4 HT in Windows ONLY, not gaming.
This is a virtual "Dual Core" and does not deliver the same processing power as literally having two CPU's on one chip like dual core in case you did not know. The problem with the P4 is it runs very hot (The chip suffers from major heatsoak, you will notice this when doing mundane tasks such as right clicking the desktop. There will be excessive latency when opening a context sensitive menu as compared to the super snappy Pentium M), requires mucho voltage, and sucks for gaming. Even the 2Mb L2 Cache P4's do not impress me because they simply run too hot. The additional 1Mb L2 Cache was a weak attempt from Intel to make a poor chip design better.
The D900t caters to a different crowd then the M570A. Most M570A folks play games, surf the net, and do school work. I think many D900t folks like the extra multi-tasking power and buy them so they can do more than just play games, people buy as "TRUE" DT replacements. The Pentium M sucks arse at multi-tasking where D900t CPU's DO NOT. It's all a trade off... The M570A is typically better for games but this IS NOT always true, look at that beast of processor available in the D900K, the FX-57, that processor is insane!
Temps vary from machine to machine, thermal compound, thermal dynamics(Case design, air flow, etc) and CPU architecture play big roles as to why one lappy may run hotter or cooler than another. 75c is very hot, but not uncommon, especially if you have an AMD CPU. You can apply some ARCTIC SILVER 5 Thermal Compound to the chip and get those temps down quite a bit.
I OC'd my machine to push the envelope only. I simply wanted to show people you can achieve these scores without invasive mods and hacking. I have since returned it to a completely factory state. 6800 3D marks is fine for me in most games and I have no reason to stress the system as I want it around for a bit. That is until SLi 7800 Go GTX lappy's come out or a new GPU upgrade comes around, then I will go nuts and OC the hell out of it ;-).
Hoped that helped...