I have a dell optiplex 760 sff and I'm thinking of buying a gt 1030 (probably pny or gigabyte) but I've heard that some 1030's don't work on legacy BIOS.
Is my pc compatible with it?
Yes, it should work, IMO. Most of the UEFI GPUs have support for both UEFI and LEGACY bios. There are no UEFI-only graphics cards, as far as I can recall. I could be wrong though.
There have been some UEFI only cards. I would ask at the MFG. website before buying any card. The vendors are clueless.
But you should definitely be able to get one to run in that. ZOTAC, and PNY are good about supporting older computers.