I just did this myself over the last couple of months.
Take your time, do a little research, here is a start.
Golf magazine, April 2011, V53, I4, has a decent run down of a whole bunch of iron sets.
The actual reviews are just so-so, except to show you who the players in the iron market are. But they break them down into three really useful categories, 1)better player irons for low handicappers, including blades, 2)game improvement irons for 7-17 handicappers, and 3)max game-improvement (14 and up).
I am guessing you are not in group 1. Do you stay under 100? If so, you might look at group 2, otherwise likely group 3. The trade off is usually group 3 forgiveness for group 2 feel. Group 3 clubs tend to have bigger faces with a lot more perimeter weighting to allow off center hits to work out better, and sometimes some group 3 does some funny business with the sole and changing it from long to short irons to try to avoid fat hits. To better players this tends to feel mushy. the super forgiving clubs dont give all feedback about off center hits that group 2 clubs do. When you hit a group 2 (or 1) club in the center of the clubface you get that golf nirvana feeling of the perfect hit and the ball does really good things. Also, it tends to be easier to work the ball with group 2 clubs (which also means it is easier to put unintentional spin on it, ie, slice.)
Stay away from graphite for your irons, fine on hybrids and up, but that tiny bit of extra power is not worth the loss of control or the extra money. My only firm bit of advice - no graphite - unless you are 65 and/or have some physical impairements. I made this mistake some years ago, and really regretted it, even with stiff graphite.
Get fitted? Even Dick's will do this I think, if not, golf galaxy (pro shops are pricey). Its nice to have clubs that you are confident are just the right height and lie angle, and they might fit you to one the groupings above as well. Not crucial, but why not.
If you are getting serious and getting better, I recomend going group two even if your game isn't there yet. Remember hitting an aluminium baseball bat in the cold? An off center hit should feel a little like that, you should know it in an immediate, physical way. The super forgiving clubs take that feeling away, while the group 2 clubs preserve some of it and still provide a good deal of forgiveness for the actual ball flight.
Get hybrids instead of long irons. If you dont break 90 the longest iron in your bag should be at most a 5, probably a 6. Hybrids work.
Once you get some idea of what you want, shop around, I almost always buy last years model. Used irons are perfectly fine, hybrids might be a little trickier, but anything over the last 3-5 years should be ok even with hybrids.
also, ask a club guy (equipement guy at golf galaxy) to measure the loft on your PW and AW if your set has one, you want to make sure you go down in like 4-5 degree increments from your PW to your most lofted wedge, that is PW, AW, SW, LW 46,50-52, 56, 60. this varies, but know what that first wedge is and don't have a huge 6 or 7 degree gap, because then all the sudden you will have no easy swing for some yardage that should be in the scoring zone.
I just got a set of mizuno's (got real lucky, saw them at a used sporting goods store totally mis-priced, $80, I took a small chance because I didn't really know what they were other than I have always loved mizuno's, they seemed a little too blade-like for me (extreme group 1), but they have enough perimeter weighting that they are great) I would have spent at least 6-700 hundred for a set of mizuno's so i am thrilled.
good luck, remember its in the dirt.