I'd replace the front tires ASAP... losing a rear is no where near as bad as losing a front when at freeway speeds...

I'd then replace the rear tires when your tires are due for rotation (I rotate mine when I change the oil... every 2-3 weeks (2500-3000 miles))...

if you don't put a lot of miles on your car it may be best to do all 4 at once and keep your good tires just in case you have hard times & a problem in the future...