If this is your first DSLR... buy the D3000... Its that simple and here's why. If you are new to photography, or even just new to DSLR, its going to take some time to get used to the camera and learn the settings. More advanced cameras can be extremely overwhelming to people who are new. If you were thinking about the D5000, I would tell you to save alittle extra cash and buy the D90, which is a far superior camera.
Bottom line, you can get a D3000 for a great price nowadays, and I don't think there is much of a fall off between that and the D5000, the 5000 has live view, and video... (and maybe alittle better burst... I don't remember) Anyway, I personally wouldn't use live view or video... So be that as it may, buy the D3000, and in a couple of years, if you want to upgrade, you can sell the body on ebay and use you existing lenses...
Not to pick on your logic, BUT:
As you spend more money on your camera body, the manufacturers start moving controls to enhance the control (OMI for techs) of the camera. Why you spend a good amount of money a camera, you expect it's operation to be smooth. The D90 for example has two control dials, Focus modes, ISO, WB, file type, etc, out on the body to keep you from having to dig through a menu to change simple settings.
To put it simply, the more money you spend on a camera body, the easier it will be to operate it.
There is a big difference between the D5000 and the D3000, the most obvious being one has a usable ISO range, and one really doesn't. The CCD sensor in the 3000 is much less capable then the D5000.