I got myself the nvidia 7300 GS graphics card but wen I put it in2 the PCIE slot and plug my monitor 2 it, there's no display. I have the 915GV chipset with a 16X PCIe slot. I did not disable my onboard graphics...could that be a problem? Do I have 2 uninstall my onboard drivers before putting in the new card? Plz help!

Yes, you need to uninstall the onboard before installing the new card. It varies from motherboard to motherboard as to how to go about doing this. Your user manual will have the details.

Usually you need to go into your bios and disable the onboard graphics as well. Some older motherboards actually require you to move some jumpers as well.

Also, it is probably a good idea to start your own thread

Could be the psu. I would try another psu just to rule it out if for no other reason. You can have a 350 watt psu that will start the machine and a 450 that won't start it. Depends on the quality of the psu. Not sure emachines uses the best of quality when it comes to this...
Worth a try..!

The emachines t6410 only has a 300W power supply. Just barely enough to handle the stock components. You add anything to the computer, even a wireless card, and it can stop working. So just upgrade the psu and it should be fine.
If not, return the psu you bought and get a new computer instead