I wanted to mention a few more options for those of you that might be seeking a lower cost of entry into version 3.5 of the D&D rules. Mongoose Publishing offers the Mongoose Pocket Player's Handbook and the Mongoose Pocket GM's Handbook, which contain most of the essential rules that you will need as a DM. Both books are softcover and retail for $20.00. I own the Mongoose Pocket Player's Handbook and it is a useful backup reference. It is also light and compact, but it does lack the durability and completeness of the full v.3.5 Player's Handbook. I would still recommend, especially to a new DM, that you buy the 'official' books, but I wanted to make you aware of the alternatives.
This brings up the subject of the Open Gaming License (OGL), the d20 System License and the System Resource Document (SRD). To make a long story short, when Wizards of the Coast published the 3rd Edition of the rules in 2000, they decided to 'open' the system up and allow anyone to publish content for the game, as long as they agreed to certain stipulations. These stipulations are contained in the Open Gaming License and the d20 System License. The content that could be used is contained in the System Resourse Document and it is this document that was used to provide the content for the two Pocket Player's Handbooks. The Resource Document does not contain ALL of the content that can be found in the official books (and that is an important distinction), but you could actually run games using only the SRD.
You can actually browse the SRD for free in a nice hyperlinked format online at the Hypertext d20 SRD. This is a neat site and contains not only the core SRD rules (from the three core rulebooks), but also open content from other rulebooks. There is also another online version of the SRD available at www.systemreferencedocuments.org. You can also download the SRD for free HERE as a series of files in RTF format from the Wizards of the Coast website for reading offline.