The 6850 is ok. Although, if you look at the minimum requirements for sims3, worse cards then her integrated card are on there. So, buying that graphics card probably won't fix much I'm afraid, as the minimum requirements suggest the problem lies somewhere else.
I've been running amd dual cores for quite a while and I was very disappointed with it all that time. When I made the switch back to pentium when they came with their new dual cores and quad cores, the difference was so immense, it made me even more disappointed with AMD.
Another thing, vista sucks and that's not the fanboy talking in me, it really sucks.
When's the last time she reinstalled windows on that computer? She should run 64 bit on that computer to get the max out of the last gig of ram anyway. If she doesn't wanna step back to xp 64 bit, I'd say try installing win 7 64 bit on there. I don't know how the machine would be running that, my guess is better then vista as vista is the worst
My guess is that she is already running that vista install for over 2 years now? 
Anyway, reinstall windows, install the propper graphic drivers and see how it runs then.
