Since you're looking for optimization for speed, that will go more likely to the hard drive. Hard drives are tipically sorted on two actual general options. 5400RPM hard drives are cheaper, and generally you get laptops with that. (Watch out since you may pay lots of $$ for a big sized hdd with slow speed.)
7200RPM hard drives have a better speed, but they've a more expensive price.
SSD drives are something totally different. SSDs make your boot amazingly faster, (watch some youtube videos of comparisons). SSDs are very reliable, make no-noise. However, SSDs are way too expensive, and they usually sort small capacity drives.
You should check what are your needs in order to know what kind of drive you need.
On the other hand, ARM processors have some architectural limits, as far as I know, Ubuntu on ARM will not let you get flash-video and other vital stuff. Still ARM is a very hard-to-warm processor. That's why tablets don't have fans, like laptops.
As for today, Windows 7 will not run on any ARM processor. (Win8 is supposed to do so.)
Common processor architectures, (x86, for 32-bit, x64, for 64-bit) are the tipical and most recommended choices. They are compatible for anything in general. Also they are SUPPOSED to be faster, (might as well be that ARM processors usually have low clock speeds). I recommend you getting a x64 processor, either from AMD or INTEL.
If you need anything except hardcore gaming, go INTEL, they architecture is proven to be a little step ahead to AMD's. However, AMD's crossfire technology (make the AMD processor work hand to hand with a ATI graphics chipset) is something to considerate).
If you do worry about heating, AMD's brand new APU (combination of graphics card and processor into a single chipset) is really great at keeping things cool.
But you now, over every actual processor, (Intel Core i' series or AMD A' series), most speed for opening things comes more from the hard drive on your system.
I know it may be confusing, but I just tried to explain everything so you can go for your best choice. Good luck! ;)