Originally posted by Wes Tank
615k's are just better then what you had on them. Nitrogen wouldn't matter ride wise. Older tires dry out and harden with time. Old tires were probably out of balance as well.

This.

No name tires tend to be much harsher, noisier and harder to balance. Getting them off the car and putting the nice freshly balanced Falkens on would make the biggest difference.

Nitrogen doesn't matter other than it has much less thermal expansion/contraction than standard air. It also doesn't leak past seals as easily. What this means is that you shouldn't need to check your tire pressure as often (but I still would)