Nope, I'd say that a 7900GT should sit nicely with a 3500+.
While performance would of course scale upwards towards the FX series, you won't notice a huge difference... and alot of modern games will be quite GPU-bound anyway.
By the time your CPU is becoming a significant bottleneck, it will be time to upgrade that graphics card anyway.
I'm sure the 3500+ and 7900GT will last fine until your next build with DX10.