As long as your power supply supports it get whatever videocard you can afford.
It’s performance will be slightly limited by the PCIe lanes but I suggest that you will reach CPU limitation before that’s an actual issue.
In short a video card is your best bet but buying one to match your older CPU better only means you’ll want to buy another later.
Get the best you can and start saving that grand or so for a motherboard/CPU/RAM … and probably power supply when you can.