I guessed as much. Which means you wouldn't be able to play L4D2 very smoothly, if at all.
Upgrading your graphics card to the best available for the AGP system won't help too much I suspect, even with everything turned way down....
Anything that would plug into your current system would not be value for money; performance/$ would be very low, plus (depending on exactly how old your system is) the fact that you may start to have "aging" problems with your computer. Components like the CPU might have a long life, but motherboard capacitors don't; PSU's have caps which fail with time too; and don't forget HDDs, which can fail even when new are much much more likely to fail after 5 years.
So unless you absolutely cannot wait to upgrade and can't afford anything better, or have some program with will run only on that PC (or some other reason not to change to a whole new PC), I suggest you get a new one.