Puke16,
Typical Op Specs require a Takeoff Alternate (TA) when you cannot return to land at your departure airport. So, if your airport has a typical ILS, you will need a TA when the weather is below 200' / 1/2 SM.
As far as Takeoff Minimums, that depends on your Company's specific authorizations. I worked for one company that was not allowed lower than standard takeoff minimums so we had to have 1SM of visibility to go. But, if your company is authorized for Lower than Standard Takeoff Minimums, then you can probally go as low as 600 RVR depending on equipment available.
For the most correct answers, refer to your Ops Manual and Ops Specs.
Hope this Helps,
JetPilot500