Chrome graphical glitches in Yosemite
After seeing it in Firefox too, I decided it must be hardware-related. To avoid having to open up the Mac and change out the graphics card (which required a bit of effort to get the new card in there in the first place, due to it not aligning precisely with the PCI slot opening in the back), I tried changing out the cord. (I recently switched from having it plugged in with a DVI output -> DVI cable -> DVI input on monitor to a Mini DisplayPort output -> MDP/VGA adapter -> VGA cable -> VGA input, and the cord has issues I guess.)

It seems to have resolved the issue.