Okay, ill cut the bs to case. I just bought new videocard since i had last 32MB inside built and i were tired for it, now i have 9550SE but problem is that when i start playing, example: WoW, Lineage 2, EnemyTerritory, GroundControl 2 so on...I can get into game but after random time 5 secs - half hour my screen is going to black and 10 secs before shutting down the whole system, like hard reboot, with no warnings, at all! I have Asus A7V400 Motherboard and it's fully AGP x8 Compa. 256 + 516 RAM DDR 400 ( tested with only one of them inside with both ) Amd Sempron 2800+ and everything should be normal with my computer, no problems occured before, atleast like this, latest directX drivers also and latest display drivers also, and i really wanna enjoy the games without lag :\ could anyone tell me where is the problem?

Sounds like an over heating problem, the PC is protecting itself by switching itself off and thus causing random reboots.
Get some motherboard utilities or something like SiSoft Sandra to tell you the temp for CPU/System for your PC, then get some more fans etc to cool it more.