You need to speak with an attorney well-versed in estate planning and the law of your state (the state laws applicable to estate succession can vary from state-to-state). At the very least, you would want a will, which allows you to set out how your property passes to your heirs. You can effectively set up it up any way that you want and have your property pass any way that you want (sometimes if you are trying to bypass a spouse, there may be issues, but that is something that an attorney can cover with you if applicable).
If your estate is a bit more complex, or you wish for your estate to avoid probate as much as possible, then you may want to ask about a trust as well. The main benefit of a trust is that the assets in a trust will pass separate from a will and you do not have to go through the probate process for assets in a trust like you do with a will.
I would also be sure to include a living will (telling your loved ones what you want done in certain situations with regard to your health - for example, you can "pre-select" that if you are determined to be brain dead and can only survive with food and water being provided you would want to be removed from life support...or not. That is the good thing about a living will - you get to lay out what should be done in different situations so there is no ambiguity or argument); and health and financial powers of attorneys that will allow someone you appoint to make health and financial decisions on your behalf if necessary.
Life insurance is one thing that can be used for estate planning purposes, but you want to be careful about the type of product you buy, etc. And also know that the death benefit will generally be included to increase the value of your estate, which could result in estate tax implications, if you have an otherwise larger estate.
These are all things that you should talk about with an estate planning attorney. S/he will be able to answer all of your questions, look at your situation, and help devise the best estate plan to make things as efficient as possible upon your death.
As for a specific amount of assets, I am of firm belief that no matter what, everyone should have a will. If you die without a will, there are state laws that will control how your property is distributed, and that may coincide with your wishes, but it might not in every case. A will can help avoid this issue, so again, regardless of whether you have a $10,000 estate or a $10,000,000 estate, I would suggest a will at the very least.
I would also suggest a living will (so that there is no argument amongst family members about what should be done in certain situations - i.e. you don't want to put to heirs in a Terry Schiavo-type situation) and possibly powers of attorneys (so if you become incapacitated for some reason, the person you appointed could make necessary decisions on your behalf).
A trust can benefit an estate of any size since it can help avoid probate; however, you can make an argument that a trust is somewhat superfluous for smaller estates. That is something that you and your attorney can discuss.
Best of luck.