Unless you have some experience with LED soldering, and a steady hand, don't even consider buying any LEDs that are not mounted on "stars", which are actually heatsinks, with solder pads on them. Even those are very small - just the LED alone is extremely tiny for doing any soldering on, and the margin for error is very small too.
You can buy those constant current drivers for 350 mA if that's what you want to use. They all put out a specific maximum voltage. The number of LEDs they will drive in a series circuit is that maximum divided by the voltage drop of the LED at the current it will be driven at. If you want to drive at 350 mA, you can also use two parallel series strings, so that each string runs at half of 700 mA - a constant 700 mA driver. There are advantages and disadvantages to doing that.
Deal Extreme sells mostly star mounted LEDs, so look there for the specific LED you want to use. Other sites do that also.
If you want a dimmable light, you can't easily use a constant current driver. There are dimmable ones too, but they are more expensive, and take more knowledge to hook up. You don't need a dimmable aquarium light - virtually no one uses dimmable lights on their tanks.