My compuer screen flashes black when im around alot of details like when im on wow?

when i play wow and im around a spell caster or soemone that dose something like fire ball frost bolt or something with great detail my screen flashed black. i have the following ram and such... Size: 320GB4 SATA Hard Drive (5400RPM...4GB3 Shared Dual Channel DDR2 (2 Dimms)...Operating SystemGenuine Windows Vista� Home Premium Edition SP1...Intel� Pentium� Dual Core T4200 (1.73GHz/800Mhz FSB/1MB cache)... please help me