ok there is something i dont understand these 2 commands both i write them in the cmd in the pc or the 1st in the pc the second in the N900 or what exactly? and what exact version do u recommand me to flash ( my N900 was bought from US ) thanks

All of this is done on the PC (except for holding down the U key - that's on the N900). You'll want to download and flash the latest version of the US firmware - it's clearly labelled on the site (RX-51_2009SE_3.2010.02-8.002_PR_COMBINED_002_ARM.bin).

now my led flash is green so am ready to flash it right i mean that means that my battery is full ? .. btw i press the U the moment i plug the n900 to the laptop ? and i keep it pressed till when ? thanks

If the light's green then it should be fully charged, yes. And you want to start pressing the U key before connecting it, then keep it held down until you can see that the N900 has booted into USB mode (the backlight will be off, but it should show the Nokia hands logo as well as a small USB logo in the top-right corner). Once you connect it, the N900 should power on and go into the USB flashing mode.

RX-51_2009SE_3.2010.02-8.002_PR_COMBINED_002_ARM.bin this one next to it there is usa varient well my device was bought from the usa but i live in the middle east so is it better that i download the global one or its the same ? ( am now downloading the one which is varient for us)

Press U on the N900 and hold it down while you insert the USB cable into the computer. KEEP HOLDING "U" until the flasher sees the phone. This means you should have already started the flasher app with flasher -R xxx.bin -F -r (not quite sure on the switches, haven't used it in a few days).

After your N900 is flashed and up and running... stay away from the extras devel repos. Even extras testing, for that matter. Sure, we need people who can test apps, but (and don't take this the wrong way), you're not one of those people. If you can't figure out how to reflash your device, don't even know about roofs and you think all the apps go onto the 2GB drive and if you panic like a little school girl if your N900 starts acting up, then the above mentioned repos are not for you.

Nobody knows what the difference is between the variants. If your phone already had the US firmware then you may as well stick with that. Once anyone tells us what the actual differences are, you can decide whether to bother changing to the Africa/Middle East version.

if the problem dont get fixed after i reflash ? i should install the eMMC and then directly after that flash it with that version ? because i read some other forms all are saying that flashing didnt fix it they made an eMMC which is also harder :S

I think it's unfair that some people now have to go thru two kinds of flashes to really flash, but the eMMC flash isn't that bad. Yes, I would recommend you flash the eMMC and read the Wiki pages carefully (except ignore the place where it says you won't have to flash the eMMC if you have a production N900, because it is incorrect).

dont forget to try several usb ports for reflashing IF it fails to start the flashing procedure. happened to me.