Computer crash - and after restart i get blackscreen

Hello ,
for the last 4 days .. my computer stucks alot while playing games ..
just crazy colors appears and then my computer restart ...
at the 1st time happened , i just logged in again to windows and kept playing .
but yesterday , while playing a game .. my computer stuck and after self-restart ... it showed me black on the screen and nothing happened while my pc is turned ON.
i cant see any thing ... no Motherboard logo no nothing on start .. JUST a turned on computer with no screen .. i sent my computer to friend he changed the GFX , cpu and ram ... it didnt work .. after he tried that with no result , he put all my stuff back .. and Suddenly i got a screen ... i took the pc home and turned it on ... i succeed to login to windows vista for 1 min and then i got blue screen with " *** STOP: 0x00000050"
and then again .. no screen after restart !! i changed the screen but its the same ...
just notice that i have 2 OS , windows vista ultimate 64bit and windows xp pro..

All this problems happened while using vista .. as i remember the partition which vista installed on has 2-5gb free space ... "95% full"

after booking the problem on the internet alot of ppl say many things .. i just cant figure it out ...
how can i slove this problem ? wt could be the problem ?
notice that i tried to turn on the computer without the RAM and the Hard disk .. i got no BEEP !! weird ?! could be the MB? GFX ?

My pc :
Asus p5q-deluxe MB
Sapphire HD4870x2 2gb
2X2gb Dominator DDR2 1066mhz
Inter Q9550 Quad core


Thanks , and sorry for bad english ...

bumb ,, soz :X but plx help im QQing here :X new pc