so if you have 4gb total, how much do you have allocated to your onboard video? Sounds like you have 3gb system and 1gb video.
Mobo: Biostar TA880GU3+ uATX
Proc: AMD Phenom II 960T unlocked/6 cores @ 3.5ghz
Ram: (4) 4gb GSkill 1600mhz (16gb total)
HDDs: (2) 640GB WD AAKS, (1) 750GB Seagate Ext
Vid: Sapphire 6870 1gb DDR5 1000mhz core, 1100 mhz mem