New Yellow/Dead Leaves

I've been searching through the threads and saw some people with leaves that were yellowing but none of the pictures were exactly what is happening to mine. That and all the pictures were plants that were much further along in the process. Anyways, I got some plants that are roughly 3 weeks old, and up until now they were growing like champs but recently all of them started their next tier of leaves and each one the new leaves turn yellow in a day or two and pretty much look dead. I haven't changed anything so why they went from lush green to a sprouting dead looking leaves I have no clue and was hoping someone might know whats up, thanks for any info.

Oh ya, the picture I uploaded is a couple of days old but shows what I'm talking about, the leaves in the picture are definitely more yellow/dead looking than in that pic.

And a side note, if someone doesn't mind throwing a little info my way, how old before the plants can be transplanted into a new pot? I asked this someone once before on a different site and the only answer I got was "You'll know when they outgrow their pot" so that did me jack shit. I did this once when they were about 2 weeks old and they all instantly died so I don't want to take a chance with this batch.

Those cotyledons (the first seedling leaves) are probably just dying because they're no longer required.

Also, that seedling looks really stretched. What are your lights?

How big's the pot?

I mainly use 400w HPS but its jackin' up my electric bill so every once in a while I use a 120w fluorescent. I'm also doing that because it heats up and I can't keep the heat low enough right now, haven't splurged on a good fan yet.

The pot is about 3 inches or so in diameter and about 5 or 6 inches high. To small?