Advice for External 5v Power or common ground?

I'm using an Arduino Diecimila to control a DAC (ic) which controls a separate project. I'm thinking I need a common ground for the DACs to work properly, so....

The 'other project' is a USB device and uses the power from a separate PC. I want to power the arduino using the other project's supply, but the problem is the VIN on the arduino takes 6-12 volts and the usb only supplies 5v. Is there a way to externally power the arduino using a 5v supply?

What if I powered the arduino using the USB from my PC, and connected the grounds from the 'other project'?
I'm thinking that would end in a nasty ground loop for the DACs or worse. I might end up trying it because I'm desperate. Is there a reason I shouldn't try?

What if I powered the arduino using a wall-wart and then connected the grounds?

Any suggestions would be helpful.

For a start you do need a common ground between the two systems.

The arduino board (what type are you using?) has a built in voltage regulator but the chip itself works off 5V. This can be supplied direct from the USB lead, or you can get it from the other project. There are jumpers on the board you can change to change the source of power to the board. Refer to the schematic of your board for details.

thanks for the reply... As I stated before, I'm using an Arduino Diecimila.

After looking at the schematic, I think I've narrowed down my search. I want to put 5v from the other USB device in VIN, but since VIN is regulated by the mc33269d-5.0, I'm unsure of the results (5Vin -> 5Vout). If that doesn't work (Vout is too low), I'll try to put 5v directly to Vcc (aka the 5v pin) with some kind of currently limiting resistor.

So my question is, is this a bad idea? Especially since I'm applying 5v to Vout side of the mc33269d-5.0.

Any better suggestions?