What kind of 50mm prime lens will work on my Nikon D5000 and also on a D90?

I have a Nikon 5000 and at a later time plan to upgrade to a full-frame camera like D90. If I buy one of those G lenses without the aperture ring will they work on D90? Also, will a lens with an external aperture ring work on D5000?

The Nikon D90 is not a full-frame DSLR. The least expensive full-frame DSLR in Nikon's current lineup is the D700, which has an MSRP of about $2, 700.

All of Nikon's current DSLR's are compatible with the G lenses. Most of the lenses I have for my D90 are G lenses. You can use lenses with aperture rings with your D5000; however, you'll have to leave the aperture ring locked at f/22 (or whatever the lens' minimum aperture is) because aperture is controlled by the camera body. If you set the aperture ring to anything but the minimum, the camera will give you an error message.

Something you should keep in mind when you're shopping for lenses for your D5000 is that it does not have an autofocus motor built into the body. If you want to use autofocus, you will need to purchase AF-S lenses because they have focus motors built into them. When you upgrade your camera body, it will not be an issue because all of Nikon's more expensive DSLR's (including the D90 and D7000) can autofocus with both AF and AF-S lenses.

The best 50mm prime for you at this point would probably be the AF-S 50mm f/1.8G.

The D5000 needs AF-S to autofocus. Nikon's new 50mm f1.8 af-s would work on it. It would also work on the D90 (which isn't full frame) it will work on the D700 and the D3 series which are full frame.