Your current CPU is kind of too weak, and it might create a bottleneck, if paired with a high-end GPU. You need to look for some entry-level GPUs. What's your budget, btw ? Will you use the card for GAMING, or some other purpose ?
Get a GTX 1050 or 1050 Ti, and increase the RAM if possible. In some gaming situations, the GTX 1050 will bottleneck the old Athlon II X4 645. But in those games that are mainly GPU-limited, the card will be fine.
I still have an Athlon II X4 650 and GTX 1050 Ti system that I drag out occasionally to play with. It games fine at reduced settings.
Your 4GB system RAM is going to be a problem tho with any card that has 4GB VRAM. In which case, the 2GB GTX 1050 may be the better choice if you can't increase the system RAM to 8GB.