Vga onboard emachine t5212 (ati radeon xpress 200) not good for video editing !
u can search mobo with HD4200 good for video editing / play blue ray
if u want full upgrade get motherboard support crossfire / SLI
The biggest issue in your case would be the quality and output of your power supply. To upgrade the video card you need at least a 250 watt PSU, even with a lower end video card, or you may run into stability issues.
If you can meet the power supply requirements I'd suggest getting the Radeon 4650, it has pretty low power requirement and should match up well with a lower-end system while being capable or running many games at good settings. Look here http://www.tomshardware.com/reviews/gaming-graphics-rad...
There's no need to upgrade the motherboard, when you have an open x16 slot already (unless the OP is looking to xfire or SLI).
I agree with hang-the-9. A low end 4xxx series card is pretty cheap, low power, and is HD. I would also add, that your PSU may not have a 6 pin PCIe power connector. Before getting a video card, see if it requires one, and check to see if your PSU has one. The video card may come with an adapter, but you'll want to make sure.