Speed vs Quantity?

1.DDR2 or DDR3?
2.Speed or Quantity?
3.Does rams that have a speed over 800mhz generate heat a lot which causes system failiure?

well for starters some idea as t what you plan to do with the machine?
overclocking? mild/medium/hardcore
what motherboard?
what CPU?
Power supply?
what OS do you plan to run? and what do you plan to do with it... gaming? CAD? Video processing? or just surfing the web?
before any advice can be given... no expert will give any without enough information...

myself I do a lot of code development, video processing, Virtual Machines as well.
Using 4 gig of ram in Vista64 is enough at the moment (all apps i use are 32 bit) 2x2GB sticks.
I went DDR3 for future proofing. Nehalem systems will use DDR3 and I will go there eventually...
the latency that DDR3 has isn't that important then the lower latency DDR2 with your CPU...

as for Mobo, any thought weather to have crossfire support? if not then X38, X48 isn't that necessary...
P45 or P35
but best to figure out what all you want to plug into it and buy the board accordingly...

any other question....
ask away....

edit: BTW speed and quality go hand in hand. faster you go the better quality components you would want...