Left 4 Dead 2 is built on the Source engine. You could max it out on an MX440 64mb.
Regardless, a GTX260 would last you for a while yet, but I would definately not make the change just for L4D2, id be surprised if you noticed any difference over your already capable 8800GTS.
I've used both in L4D2 and can tell you a 260 is a worthy upgrade. However make sure your processor isn't utter crapola first as the source engine loves CPU speed.
You say that, but I have to run L4D2 on Med - High on my ATI X1900XTX, and still get FPS drops in areas like the park, L4D runs fine though.
Jcb33.