I don't know about marginal. It gave my computer a pretty significany speedup. I went from ~8000 on AquaMark3 to ~26,000. I can play HL2 on high settings instead of low, and I meet the minimum requirements for a few more modern games, although my primary interest is games from around '05, which the 610 I mentioned can easily handle. I ended up spending $100 in total on this rig so I could get into older 3D games, which I think is pretty good.
Of course, you're right about the PCI. If you have anything better than a PCI slot, you should use that instead.