Are you sure it only has an x8 slot? Some information I looked up online seemed to suggest it may have a PCIe 2.0 x16 slot. Have you checked inside the system to see whether the graphics card slot is full-length?
However, this does seem like a fairly old system, and the CPU and other hardware may also be significantly holding back performance in the software you are using. The 250 watt power supply is not going to run something like an RX 570. A WX 4100 is low-power professional card, and might run assuming that older motherboard plays well with it, but it will offer less than half the performance of an RX 570 in most applications, while costing more.
It might be worth looking into a full system upgrade. If you are in the US, for around $500 or so you could build a much faster system, including a 6-core, 12-thread Ryzen processor, an RX 570, 16GB of DDR4-3200 RAM, and an SSD for fast storage. You may also need a new Windows license, if your existing one can't be transferred though. However, you would end up with a much more well-rounded and modern system that should run most software noticeably better than what you currently have.