Flash or Lens

[drewphotograph]

Hi all,

I am kinda confuse which i should buy first, another lens or flash first.
What do you guys think?
Which is more important? Lens or flash?

I have got 17-85mm IS as a bundle for my 400D. I'm happy enough to have that lens

Oh ya, if i want to add a lens, I would like to buy SIGMA 70-200 f/2.8 and for flash I want to buy 580EX II. But is it ok to use that flash for my 400D since when i saw that flash... wow, it's big & my 400D is small

thank you.

Btw, here's some of my photo