There is a shload of info out there concerning med kits. A lot of it is personal preference. I got my EMT certification a long time ago, and had some simple combat med training in the military, but even a basic First Aid course is a good idea for anyone. (You don't have to be an actual brain surgeon to stop somebody's hand from bleeding. ) Keeping it simple is best.
I put together a couple kits based on two levels of necessity. First is for the minor band aid/splinter/aspirin type stuff, which is just a small bag under the seat or in glove box. The bigger kit is a lot more inclusive in case I happen upon a crash scene, etc. (I carry a large medic bag under the back seat for that, along with the extinguisher. )
A lot of folks build their own kits to suite their level of expertise, but generally you'll want some gloves. (You GOTTA have gloves!) Some 2x2 and 4x4 sterile pads, band aids of various sizes, tape, and then whatever else you can think of. Couple bottles of water, ice pak, heat pak, blanket, candy bars, etc. A box of baby butt wipes for wiping hands is alway nice, too.
There's nothing wrong with the Costco type car first aid kit, but for me it leaves some stuff out, and I'm not a fan of the burn creams.
As mentioned, a fire extinguisher is right at the top of the list. You may never use any of this stuff, but it's sure beats standing around watching somebody bleed while you're trying to get a signal on the cell phone.
A good start is the Platoon medic size kit available from most surplus stores. It's got the basics, is relatively cheap ($12) and small.
http://www.cheaperthandirt.com/ctd/...gid=&mscssid=UXVCA608AFKM8LLPHAHBTCU72F4SE6WD
Cheaper than Dirt has another on special right now for $15.
http://www.cheaperthandirt.com/ctd/product.asp?sku=MKT-1018&mscssid=UXVCA608AFKM8LLPHAHBTCU72F4SE6WD
I picked up a large medic kit from CTD and another on eBay for $50 for the trunk of the other car. I'd recommend you do a quick search for first aid kit in some of the survival type boards, and you'll find more than you ever wanted to know... and they have med kit holy wars just like we have over synthetic oils and exhaust brakes... ;-)
Hope this helps.
TP