Ironic that this was posted, I was about to create a thread about which extension tube to buy.
Currently, I own a PK-13, which works fine with my 55mm f/3.5 AI-S Micro, however that's because it's an old AI-S lens with an aperture ring. The newer "G" lenses (such as your 70-300) which lack an aperture ring and the aperture is controlled digitally, need to be coupled with the camera body, so you'd need an extension tube that can transmit that data.
Personally, I'd love to see Nikon produce an extension tube that would be able to do that, but believe it or not, the latest Nikon extension tubes are the ones that you mentioned, and they're from the '70s so they're not designed to work with Nikon's new, digital bodies.
There is one brand that makes extension tubes that will meter and autofocus with the newer bodies and lenses, that brand is Kenko (sub brand of Tokina), and they make extension tubes such as their Kenko DG Teleplus Extension Tube Set, that will maintain your autofocus and metering. It costs around $180 or so here in Canada, so it's pretty cheap for the effect that it produces.
You may alternatively want to consider a dedicated Nikon micro lens, such as the 60mm AF micro that Jeremy recommended. As a dedicated macro lens, you're getting a much easier to use and versatile lens. If you go ahead and couple a dedicated macro with a few extension tubes, you can get some stunning effects, magnifying the subject up to 5 times or more, so you can get some beautiful, unreal shots of insects.
Gear: 7 Nikon Nikkor AI-S and AF-S lenses, SB-700 flash, Nikon D7000, Nikon FM, variety of accessories
"There are no rules for good photographs, there are only good photographs."
- Ansel Adams