I have a Intel 946GZ Chipset with PCI x16 support and Pentium D @ 3GHz I am currently using XFX Nvidia 8600GT 512MB DDR2. Can I upgrade to NVIDIA GT240 ?
Yes as long as you have a power supply that meets the requirements of the GT240. On the GT240 are you going with the DDR3 or the GDDR5? This link shows you where cards stand in order of best to last.
The video card is not afected by what ram you have in your computer , but the overall performance of the computer is better if you have all parts that will perform at the same level and the GT240 is not an overpowering video card for what you have.
If you were to put in a GTX 560Ti for example then the video card would be outperforming the rest of the parts and the computer would be unbalanced.