My immediate reaction to your original question would be to wait until you can upgrade the power supply. (That said, it sounds like you may have already given it a go). AMD recommend a 500w power supply for one of these cards so I think your machine will have trouble if you introduce a second, as a ball park figure I go for at least a 1000w PSU for an SLI/Crossfire Configuration, but its entirely dependent on what the user will be doing and what the other components in the system are. For you, I wouldn't risk it cause it looks like you may be doing some demanding stuff with your machine. Although the machine may not fail straight off the bat, you could be shortening its lifespan.
I tend to give machines rather monstrous power supplies, just in case they need to be upgraded or the user suddenly develops a taste for 3D gaming, So I have never really experienced an under powered machine. Though I know people that have, it can cause issues with other components, BSOD and could cause the existing PSU to fail, depending on its quality.
I would be intrigued to hear how your machine runs once you have done it (I hope its OK and I'm just being over cautious). So could you post and let us know?