Since it's a dual-CPU system, I assume it uses a server board. Given it's relatively low-spec stuff, I'd go with an older/less equipped GPU. Preferably one with no external power draw, getting its TPW from the motherboard. The GTX 750Ti is the most popular option for budget 720p builds, but a 750 works fine. In all honesty, anything above the 500 series GPUs will do you. The most popular are as follows;
GTX 750 Ti
GTX 750
GTX 5xx
Radeon RX 460
Radeon RX 560
Radeon RX 550
GT 1030
THose are really the best options for a single-slot system, or just for photography.