I agree with the others. Skip the infant carrier and go straight for the Britax Marathon. You spend $45 for the infant carrier plus another $25 for the base. And then they grow out of it in 6 months. Then $250 for a convertible carseat for ages 1-6. Then another $100 for a booster seat for age 5-8. We used Marathons for both kids. My son outgrew the Marathon in height last year so we had to move him up to the Britax Frontier 5pt harness booster seat. The infant carrier was a waste of money for us. Plus after the baby reaches about 10lbs,it's a back breaker to carry them around in the carrier. But I have no experience with twins... I have a 6 year old and an almost 4 year old. Both my kids preferred to be in the Maya wrap when they were infants when we were out and about. The more squished they were in the wrap, the better they slept :).
I would register for 2 Marathons and 2 infant carriers. If your friends and family buy it for you, great! If not, you can always go out and buy the cheaper infant carrier for now and in 6 months purchase a bigger car seat.
Now for double strollers...depends on your lifestyle and what you will use it for and the size of your car. I used the Grace DuoGlider and it was a pain...literally on my back and to steer it. It was like driving a limo from the back. Hated it! I went straight for the Bob Revolution Duallie. Works great for our active lifestyle because it rolls over EVERYTHING! I used it as a jogger and as my everyday stroller. I have also pushed around my best friends double strollers (who ended up getting 3 different double strollers because she didn't want to spend the money for the Bob, but ended up spending more money in the long run). She got started off with the Peg Perego one where you can move the seats so they face each other or face front to back. She ended up hating that one and never used it. Then she got a Combi Twin and that was good for short walks and the mall, but the small wheels always got stuck in sidewalk cracks or got stuck in the wrong wheel position so she was always bending over to fix the wheels plus there was very little understorage and the thing would tip over if she hung anything on the handle bars, even a small diaper bag. Then she got a Sit and Stand XL. That thing was so hard to push and steer and the kids were always fighting over who got to sit or stand cuz neither of them wanted to stand in the back. They always wanted to be in my Bob Revolution Duallie. So I would have 2 kids in the seats and 2 kids sitting in the foot rests and still could push and steer it with just one hand and no back ache later. I think Babies R Us now has Bob Duallies, so you can put it on your registry list. Don't forget the handle bar console too.
FYI: My daughter who is almost 4 is still in her Marathon and probably won't grow out of it because she is on the shorter side. My son grew out of his at age 4.5 because he is a very tall kid. I used my Bob Duallie from the time my daughter was born until just last month...so almost 4 years. We got a Bob Revolution Single stroller when my son was 6 months old, and we still use it for long trips to amusement parks. One kid sits in the seat and one kid piggy backs in the foot rest. Plus Bob strollers have a high resale value on CL compared to the other strollers.
Good luck to you and congrats!