You don't "need" a new video card.
However, I suspect your current card is a 1X AGP.
So, a newer 4X AGP card may have better performance.
1. If this is a SDRAM style motherboard, it is recommended to set the CPU speed manually in BIOS settings.
For some reason, many of these boards "underclock" some CPUs.
And, for whatever reason, Athlon / Duron CPUs don't work well when underclocked.
2. When you changed motherboards, you changed a lot of the devices and thus the drivers that windows needs to use the devices.
What you need to do when installing a new motherboard, is;
Go to device manager, and remove all motherboard related devices.
Shut down the PC.
Swap the boards.
Load the correct drivers for the new motherboard.
3. If you are lucky, you will get most of your problems repaired by installing the correct drivers for the chipset on the new motherboard.
You may need to re-install your video card and other drivers after updating the chipset drivers.
4. In BIOS settings, make sure that the video card related settings are correct for your card.
If you have an AGP card, you may need to set the AGP speed to 1X or 2X in BIOS. (Unless the card is a 4X card)
5. At this point, a re-install of Windows may be what you need to do.
Follow this sequence when installing drivers:
Mother board / chipset
DirectX
Video card
Other cards.
Make sure to check with the modem and sound card mfg.
Some mfgs require installing the drivers before installing thecards.
Chas