I have a Dell Dimension 3000
RAM: 512 MiB
CPU: Intel® Pentium(R) 4 2.80GHz
Graphics: Intel® 865G x86/MMX/SSE2
What do I have to upgrade to run Ubuntu 12.04 3d.
You will need a supported graphics card.
Take a look at this list
You can find a listing of graphics cards by vendor, and if 3d is supported in Ubuntu as of 11.10.
I would personally use the information to find a old (but not ancient) ATI or Nvidia card for cheap.
The requirements for Unity are HERE.
Also HERE you have a list of hardware requirements for the Ubuntu 10.04, not much should change 12.04.