Crashing in games

[reflex]

well i play wow and bf2 and about 30 mins of playing the game my graphics just freezes and my screen goes black some times it comes back somtimes it just dropes out but the thing is my pc is still on :/ well these r my specs
AMD athlon 64 3000+
1gb ram
powercolor x800xl

plz help me fix this problem its only just started happing since i went from my old 9600xt to the x800xl