I have twins and just used the basic Graco Snugride seats (I think they had a 22 lbs weight limit). They were well rated in Baby Bargains when I was looking 4+ years ago and under $100 each. I didn't go with a higher weight limit because the seats are heavier. Another thing to remember is that babies tend to outgrow seats by length before they hit the weight limits. I kept one DD in her seat until she was 1 and the other for a couple more months because she's smaller. I also LOVED my double snap n go stroller and was unbelievably sad when I had to retire it (definitely check with your local parents of multiple group and you should be able to find a good deal on a used one).
We went with Britax Boulevard convertible seats. I had reasons I went with the Boulevard, but can't remember them now. I really like the Britax seats because they are really easy to install. We kept DDs rear facing until 2.5 (although hadn't hit rear-facing limits) and at 4, they still have quite a while before they will outgrow the seats.
We also have a set of Cosco Scenera seats for our second vehicle and for travel. They are only about $40 each and perfectly safe. They are more difficult to install and not nearly as cushy as the Britax seats, but they are much lighter which is good if you're flying somewhere.
***Twin fraternal girls born at 35w6d in 12/2008***
I would go ahead and register for both. You'll need both eventually and if someone wants to go ahead and buy one of the convertible ones, rock on! One less thing to have to buy two of in a few months. Personally, I'm a Britax fan for convertibles. And what the others wrote about the weight limits on the infants is so true. We got the bigger Graco that goes up to 30 lbs and 30 inches. (or something like that). My 4 year old is only 36lbs, but he outgrew the height limit by 8 months. Go for the smaller/lighter car seats, especially since twins usually aren't huge babies to start with.
Congrats on your twins!