That's a tough question, and it's one that I've asked myself before. Are you specifically referring to JUST the talent level of your opponents ... OR ... are you considering average pot sizes, etc?
I guess you're referring to talent level. It's a tough question. A good / winning small stakes NL player is a camper. Basically, he waits for hands that will turn into a monster (pocket pairs, suited connectors), or he'll play his legitimate hands with an open raise or reraise. On the flop, this player should take stabs at the pot (ie: steal) if it's checked to him in LP by two (possibly three) or less players. On the whole, he's looking to double up.
The (6max) limit player, on the other hand, is more of a grinder. It's all about big bets and exploiting mistakes. Personally, I think the jump from $2/4 to $3/6 in the limit world is a bigger one than people give credit. I have seen that players are more aggressive, they are more likely to three bet w/ middle pair (which can be a strong move in some hands), and they are just more reckless.
Therefore, (LOL), I would say that your average $50NL player is equal to a $1/2 and possibly $2/4 L player... but I'd say the average $3/6 L player is more like $100NL (possibly $200NL!?).
As you can see, it's hard to really give an answer. That's just my opinion.
I think 2Tone would have the best perspective on this. From what I recall, he's been primarily a $2/4 (full) limit player, but has recently been clearing bonii playing cheap NL games. ...2Tone?