If you have a D70, get at least a SB-800. If you have a D80 or higher, then you'll be fine w/SB-600s.
The reason I suggest at least one SB-800 for the D70 is due to the fact that it has limited commander mode abilities. The commander mode on the D70 will trigger flashes via commander, but doesn't allow you to adjust the flash power remotely. On the D80 and D200 you can control the power of each group remotely. In the case of the D70, you can use the SB-800 as a master to trigger flashes remotely (SB-600 can't function as a master).
For the D80 and D200, the commander mode has the ability to adjust the flash power via the menu. You can also dial down the onboard to have less effect on the exposure from front lighting it via the onboard. There's also an accessory where you can attach so that it blocks out the flash from the onboard used to trigger remote flashes.
For all Nikon, there's also the option of the SU-800 commander unit. This can be purchased as part of the kit that Art has linked or independently ($250). The unit is very similar to Canon's ST-E2 unit where it will sit on the hotshoe and allow you to trigger multiple flash units. For the price, it's almost equivalent to a SB-800 unit, so you might be wondering why you would go with such an option? The reason being flexibility, frontal exposure, and range. A SB-800 on the hotshoe can trigger other flashes as a master, but it's limited to 2 groups, whereas this unit can trigger an additional group. With either the SB-800 or the onboard flash, you will get exposure from the onboard or the flash on the hotshoe (you can dial down the flash power on the D80/D200 to --, but it will still fire; it won't affect exposure on the other hand). The last one being range. If you're working in an expansive space, the SU-800's range is farther than the SB-800s and is more consistent and reliable.
Lastly, there are pocket wizards for remotely triggering strobes/flashes, etc. The price for one of these will run you about $185 and you need one transmitter and as many receivers as you have flashes/strobes. As you can see it can get expensive, but on top of these costs, there's also other things to take into consideration. You will lose Nikon's iTTL functionality and will have to figure it out (a light meter will do wonders here). What this system has over all the others is overall flexibility and range. You can shoot around walls/obstacles and from quite a distance (pro shooters will use these to trigger strobes set above courts/stadiums).
Last comment on the Nikon macro lighting kit since it's mentioned. You really don't need it to shoot macro; they're nice but isn't necessary. The best part about the kit is the SU-800 commander unit. The flash units, SB-200, comes in a pair, but you can string up to 8 of these together on the front of the lens and have a ringlight that you can shoot with. Weightwise, this isn't really an issue since they're relatively lightweight, although it might not balance correctly with a lighter body since most of the weight will be distributed to the front of the lens vs. body.
Here are some shots with the SB-800 to show you that it isn't necessary to have such a flash when shooting macro. These are taken w/the SB-800 on the hotshoe.
As for reflection off the glass of an aquarium, there are ways around it even with the flash on the hotshoe.