1) Card examples:
$44 - GT720 (very little power, basic "good enough" solution):
http://pcpartpicker.com/part/evga-video-card-01gp32722kr
$100 (after rebate) - GTX750Ti (good for light/medium gaming even with that older CPU, and also hardware accelerating some 3D applications):
http://pcpartpicker.com/part/evga-video-card-02gp43753kr
Both cards support fairly modern VIDEO DECODING to reduce CPU usage where supported. Useful if 1080p is problematic due to CPU spikes etc.
2) 64-bit Windows 10 upgrade:
*If you only have 32-bit Windows 7 or 8.1, then you can still upgrade to Windows 10.
In that case, if you are already activated then I'd burn a W10 install disc from MS site and do a clean install. For example:
1. Create USB or DVD for W10 64-bit using the media creation tool but make sure it has the 64-bit option:
https://www.microsoft.com/en-ca/software-download/windows10
2. swap graphics cards
3. backup data, write down program names etc
4. boot to install disc (can attempt to start from "startup.exe" but I don't know if you can start a 64-bit clean install from 32-bit Windows).
5. DELETE partitions (suggest no other drives attached)
6. SKIP the request for a key
7. Finish install, update drivers, add programs etc.
Other:
Probably a good idea to make a backup IMAGE of Windows regardless. Acronis True Image or similar (WD has a free version. Seagate has the same tool called Discwizard).