If you are self employed, you should seriously investigate a solo 401k. A solo 401k allows you contribute the same percentage match as the SEP. However, in addition you can contribute a salary deferral of up to $15,500.

Note: this can not be done for 2007, while a SEP like other IRAs can be setup anytime before your taxes are due. A solo 401k must be setup during the tax year, which by default is the end of the calender year 12/31.

So you can setup a solo 401k for the 2008 tax year. There is a little more involved, but most of the fund companies will help you with this.