Sometimes when alt tabbing out of wow, i get a warning there is not enough resources and to either ignore switch to a lower color scheme or continue and alert me next time it happens.

Well nothing seems to be actually wrong so whats going on?

Asus Sabertooth X58, 6gigs kingston hyperx triple channel ram.
Radeon 5870, Seagate Momentus Xt Hard drive. i7950.

No overclocks.