OK.
The D5000 body only second hand will at best sell for R5500 and thats pristine condition and a very willing buyer, I would aim for about R5000.
I have seen the D7000 Body only available for about R11500 although from the USA you can get it for about R9000, so long you dont mind missing on the warranty.
I went to
DXOmark and did a comparison between the D5000, D90 and D7000
Now I add the D90 since it sits between the 2, the D5000 and the D90 sharing the same technology with the D90 having a more full setup while the D7000 is the D90 successor.
The D7000 is by all accounts an excellent camera, however will you get to enjoy all the extra features for the amount you will have to spend?
In terms of ISO you probably never want to exceed 3200 regardless of which of these cameras you use.
In terms of IQ etc they all very close, and while the D7000 is better it does not leave the D5000 embarrassed, and is it really R5000+ better.
If you want the D7000 just get it, if you not sure you need to upggrade or what to upgrade to look at the D90, its a tried and tested camera, not the newest I agree but very good feature set and well respected.
Oh and will you take better photos, doubt it, will the camera have something extra that will make it all that much better, probably not ... just newer tech, minor improvements and a few extra buttons.
Bah, if you Really want it, and can afford it then get it.