Scar13, I have a very similar system to yours (see P4C in sig) and my 64meg Ti4200 is still a rocket in just about everything other than Halo and certain scenes in Call of Duty. I always play with details @ the max at around 1024x768, no AA, but I have 8x aniso on. This looks great to me and I've gotta have high fps.
Oh, I did overclock the snot out of my card though... if there were a Ti4500, it would have my clocks.
I'm expecting things to be slower in HL2 and Doom3, but they're not out yet, so a new card isn't needed.