What's your reason for taking it? Will you be combining it with caffeine and/or other stims?
For a simple "feel good" mood boost, I have added ~250mg to my pre-workout supps that don't have it. For more energy, you can go higher, but I have an average/below-average tolerance for caff, so 500mg is all I've ever needed.
I've never taken it specifically for it's vasodilating properties, but I suspect you'd have to go up to 1g to notice anything significant.
Here's a good read:
Invalid Link Removed