Publication 560 (2006), Retirement Plans for Small Business
is the IRS publication for small businesses such as yourself and retirement plans.
You can have a SEP IRA or a solo 401k and also have a 401k or other defined contribution retirement plan through work but there will be limits on what you can contribute. From the above pub 560: If you contribute to a defined contribution plan (defined in chapter 4), annual additions to an account are limited to the lesser of $44,000 or 100% of the participant's compensation. When you figure this limit, you must add your contributions to all defined contribution plans. Because a SEP is considered a defined contribution plan for this limit, your contributions to a SEP must be added to your contributions to other defined contribution plans.
For a nice plain language description of contribution limits when you have multiple plans, I suggest reading this article: Common Questions About Retirement Plans
I tend to favor solo 401ks rather than SEP IRAs for small businesses that don't earn a whole lot of money because you may be able to sock away more money in the solo 401k.
I wonder, but I haven't looked at the issue, if you could roll over your simple 401k into a new solo 401k which you establish for your LLC.