Do you know in what setting you'll be using them in?
That's a great budget to work with. If it's indoors/in-studio, then cassard's suggestion would be the best; get an actual studio kit, with proper light modifiers. Not only do you have faster recharge to shoot, but also the light modifiers are much stronger. If you're planning on doing on-location stuff, then speedlights would be best.
I use speedlights, because my professional work is all on-location. I've been wanting to get alienbees and a power-pack, but that would be for probably only 40-50% of my work.
Here's two simple examples of different speedlight set-up, which I posted in the "Portraits" thread:
This is only one flash in a large umbrella (60" I believe)
This is with two bare flashes on each side. I found it a bit too hot/harsh shadows, but I didn't have too much time to set-up with the horse, plus the softboxes made it feel uneasy
Very convenient, and easy to pack. I'll break down the price (I don't even know how much I spent on them):
- 2 x Nikon flashes (2 x $300)
- 2 x Vivitar 285s (2 x $60-90)
- A crapload of AA lithium-ion (or wtv they are) batteries. I haven't had to throw out any batteries in 2 years. 5 sets of 4xAA at about... $8 each when on sale = $40.
- Phottix Radio-triggers (I think it was less than $200 for 1 transmitter + 2 receivers) I also have back-up cheapo ones for like, $40-60.
- Small softbox, 2 smaller umbrellas, 60" umbrella.... stands. Probably $200?
Total: Around $1000?
Wow, I didn't even think I spent that much on it. But I do use my flashes (even on camera) all the time, so that $600 on speedlights is well-worth it.