Have to say: That is one awsome post by AdrianWerner :)

There is so many reasons to update my pc... 

how do i know if my computer support ddr2 ?

i mean, if buy a new computer and a ddr2 RAM, will be able to stick my current DDR in too ?

is the cheapest core 2 duo available better than a pentium 4 2.8ghz and just how much is it better ?

how many hard drives can i put on one cheap motherboard ?

youwho

1. if your pc is amd (old as yours), then only ddr will work.

2. No, (unless you want to broke your new pc)

3.  I would say yes. C2D offers lower power consumption, lower heat and high performance per clock than P4.

4.  How many IDE/SATA slots there is? You can but 2 HDD to one IDE cable, but only one HDD to SATA cable.