You should take her in and get her spayed now. Spaying removes all of the female organs in a cat ~ and it will eliminate any pregnancy along with all potential for further pregnancies. Having litters so close together may be possible, but it certainly isn't ideal, and it's one of the reasons animals have shorter lifespans in the wild.
If you opt not to spay now and allow another litter, feed her kitten chow for the duration of the pregnancy and until after the next litter of kittens is weaned. Adding some plain yogurt to her diet and a raw egg yolk every few days will go a long way towards replacing the nutrients her body is using up (the kitten chow has much higher calorie and protein amounts). Afterwards, even if it means confining her to a room, don't let her get out again until you can get her in for spaying. Too many pregnancies/heat cycles will leave her prone to breast tumors ~ spaying lowers that risk a bit.
at 9:13 AM on Apr. 18, 2010