Congratulations! I know it can be very overwhelming, especially if you are the first in your group of friends to have kids.
everyone is going to have opinions on must haves and they are going to be very different for each person. I highly recommend the Baby Bargains book. They took a good look at what you will need, what might be really helpful and what is just a waste of money. Also they tell you what is typically safe to buy new or used.
I don't know how big your shower is going to be, but I found that people wanted to buy the cute clothes and not the really useful things (for the most part). I think that whatever you can get used (either at a kids consignment shop, Craig's list or from friends) is a great start. Even though you are spending the money yourself, you can go a lot further with your registry if you've taken care of a couple of the big ticket items.
A few specifics
The Snap n go stroller (or there is another similar one) fits any bucket car seat and is just a basic frame. I think that everyone I know that got a travel system regrets it because they don't like the stroller that much once the baby is out of the car seat.
I'd get a bucket car seat that goes to at least 30 inches. Mine only went to 27 inches and my baby grew out of it by 5 months. Some go up to 32 inches and by that time they will be pretty heavy, but you can still pop it into the stroller.
I preferred the My breast friend pillow to the boppy for breastfeeding
It is nice to have a small changing set either instead of or in addition to a diaper bag. It has a changing pad, spot for diapers, wipes etc, but it is small enough to fit in the stroller or another bag.
If you want to carry the baby, a pocket sling like a hotsling or a wrap like the moby wrap is great for newborns and a bjorn is great for when they get a little older (I think a lot of men are more comfortable with a bjorn than a sling or a wrap)
If you are going to pump, they strongly recommend not getting a used pump, so it is a good item to register for. If you have trouble nursing some health insurance companies will pay for a pump or a pump rental.
a pack n play with bassinet and changing table. Great item to get used. I got two off of Craig's list.
A Glider chair, bouncy seat, exersaucer, bathtubs and swing - great items to get used.
I'd ask for rechargeable batteries and a good charger that charges all sizes of batteries. You won't believe how many batteries you will go through.
It was important for me to have the things I'd need for the first month or so of my babies life. Clothes, diapers, wipes, a place to bathe (sponge baths only for the first few weeks) a place to sleep and a place to change etc. Once you get those covered it is nice to have a few things you will need down the road.
Somethings that I personally think are unnecessary.
Diaper Genie, some people love them, but I didn't want anything that I had to buy brand specific bags for. We just got a covered trash cans and use shopping bags.
A wipes warmer - some people swear by them, but for the life of me I can't imagine why you would want to get your kid hooked on that. What happens when you are changing them away from the house?
A changing table, depending on what furniture you have one of the changing pads on a dresser makes more sense to me. Although some changing tables basically have a top piece that comes off so you can use it as a dresser later.
Some sheet sets come with bumpers, valances, etc etc and can cost a ton of money that I think is better spent elsewhere personally.
newborn size clothes are too small for many babies. If you want to have some on hand save the receipt and don't wash them until after the baby is born if he/she is small.
Remember, Babies R us only lets you return things for 90 days (I believe) so unless you are pretty sure about the items for when the baby is a little older you might want to return them and save the money until then so you know just what you want. Also, I registered for a bunch of stuff that was only online and people like to shop in the store (plus the shipping is really expensive on babies r us) so I didn't get a lot of what I wanted/needed.
anyway that is my 25cents
Good luck, relax and try to get some sleep!
D.