Congratulations! We're expecting our 2nd baby boy in December too! :)
A couple things re: stores-Babies R Us used to be very liberal with their return policies (I used to work there)but while in line I've seen that they've been kinda cracking down on the 90 day rule lately. They will take things back with a receipt during 90 days, but I THINK they'll only give store credit for what the current price is if you don't have a receipt. I don't mind that, but you may want to phone them and ask to be sure. When I was pregnant with my previous baby, I remember being very upset with Target because of their return policy. I can't remember the details, but I know it was harder there than anywhere I'd ever been in my life, so I won't shop there now. Some people from Jeremy's work had bought size 2 Target brand diapers and sippy cups or something, but my son was a preemie so size 2 was a very long way off, and we wanted Pampers swaddlers not Target brand, but the store gave us grief about returning it with no receipt. I was upset because OBVIOUSLY it came from there--it was their store brand! They copied my drivers license and said they would only allow a return 1 or 2 times in a year, something silly like that. Don't stock up from that store, lol. I only go there the day after Thanksgiving for a sale now, but nothing more.
You got good advice re: clothes. What I've done is buy 1 pack of onesies and 1 pack of footed pjs for both sizes N and 3-6mo (Until I see how big he is). You'll have to pay attention to your baby to see how he is: I'm warm natured but my first took after his dad and is cold natured. I LOVED the 2 sleepsacks I got at Babies R Us-they're sleeveless to prevent overheating, but much much safer than a blanket. I remember I swaddled him when he was new, and as he got a little bigger those sleepsacks were awesome. To take the baby out, I'd put him usually in pjs (sometimes a onesie underneath to protect his chest), his little cap, and wrap him in a light receiving blanket. Then I'd bundle him in a thicker blanket to shield him from the wind on the way to the car or into a store, but then remove the big blanket once inside.
For diapers, I have 1 box of size N and 1 box of size 1 until I know how he'll be growing. I love the idea of the giftcards and will incorporate that. For formula, I tried very hard to breastfeed but just wasn't producing enough to feel ok....my first was a preemie and I refused to let him lose any more weight. So I would pump, then supplement my breast milk with formula as the hospital showed me, for 6 weeks and then I just let him do formula. We did Enfamil for awhile, but he had terrible reflux. It stopped immediately when we switched to Good Start though. I will try breastfeeding again, but I did buy 2 canisters of Good Start "just in case"---a security blanket for mom, I suppose. They both expire June 2010 so I feel ok with that. But with that: this is weird, but with BOTH brands of formula, I noticed that no matter what the expiration date was, it just seemed fresher if bought at Babies R Us than Walmart. The powder wasn't as fine and it just didn't seem exactly the same from Walmart. I don't know why, and I know that sounds stupid---they were the same brand, sealed, with good expirations, but there was a little difference. So I only buy formula from Babies R Us now, though I wouldn't be adverse to trying Sams (thinking of getting a membership there). I thought I was stupid or imagining things re: the formula, but I showed my husband and a friend and they were surprised to see the same thing. I think giftcards for the diapers and formulas are great ideas, and really convenient. Just determine what the formula/diaper cost is that you're wanting to spend every week and purchase a card for that, once you have an initial stock that you're comfortable with (I want at least 1 of everything in case I can't get to the store or the weather is bad, ya know?). Again, congrats on the baby and thanks for asking the question because I liked the gift card suggestion I learned today.