If the sagging is severe the zip ties are actually a pretty good solution. Not a lot of computers come with them anymore, due to video cards and board thickness mostly, but there are some chassis that have a card slot holder attached to the drive cage. It is one of the reasons many new style chassis have the PCI cards vertical.
If it is minor it just needs to be monitored. If you notice tears or texture issues in games, sometime it can be caused by slot misalignment and intermittent connections.
Simple solution is to turn off the computer and lay it on the side (or so the motherboard is facing up) loosen the motherboard and PCI slot screws, and scoot the motherboard around until the sagging card looks perpendicular (or use a square if you have one). Tighten all the screws and everything should be fine.
Thermal expansion/contraction will ever so slowly move things around, so you may have to repeat this every year or so.