Is that an M-ATX board in an ATX case? Nothing wrong with that, just a bit weird.
Never heard of Mercury before.
Im guessing by the IDE connector its a 775 board. Do you know if the RAM is DDR2 or 3? Or what CPU is in it (CPU-Z should tell you).
Whole thing is fairly nondescript. Only branding is that Mercury logo, with no printing to tell what each slot is or any of its features.
Heres Mercury's website with a search function for their motherboards. All the 775 mobo's listed dont look visually like yours, but that might not mean much.http://www.mercury-pc.com/p-mainboards.php
In a nutshell, its gonna be pretty hard to trace the heritage of that mobo. I would just stick in the new GPU and hope for the best. As I said before, those cards aren't big performers, so I doubt a bottleneck will occur.