It was worth it for me to upgrade my videocard despite having an older CPU/Mobo..
I'm running an AMD X2 4800+ / 2GB DDR1 PC3200 / EVGA GTX 260 (Superclocked 216 core model) and yes the CPU is a bottleneck but upgrading from a 7800 GTX 512mb to my current GTX 260 still gave me a noticeable boost in performance.
Bottleneck wise, for example, Ive been playing Left 4 Dead 2 demo lately and although most of the time it runs 60fps (and a lot higher when not much is going on) with all in game graphic settings maxed there are times when the CPU's bottleneck sticks out like a sore thumb, such as when a pipebomb explodes and body parts go flying everywhere the framerate drops to 30fps during this time, which is not a huge deal.
Some pics I took with FPS counter active:
Pipebomb explosion (hello bottleneck)
Carnage
Idle
It also made Crysis playable with High graphic settings (but not quite maxed settings), It ran horrendous on the 7800 GTX.