SebM, I think your choice is a good one. Battery capacity is definitely not an area to skimp on. Too little is bad; extra reserve is good.
Hangtimes Hobbies ("NoBS Batteries")
http://www.hangtimes.com/sportrxpacks.html has some good info about selecting batteries. They typically recommend NiCad or NiMH cells. The site discusses impedence numbers and how that impacts voltage drop with lots of current draw. With the S3004s it probably doesn't matter as much as it would to someone running powerful digital servos.
Most of my packs are from Hangtimes and they are the 1650mah NiMH, 4.8 volt, variety. I use "standard" ball bearing servos, and nothing I have uses more than 5, so I have plenty of capacity for a day at the field. I still check them (in case something bad happens) but so far it has just been for peace of mind.