However, I wonder whether the ti4600 that I already have is the best card
for my current system or could I gain a little extra performance out of
it via a slightly newer card.
You could gain better performance at higher resolutions/antialiasing levels
with a newer video card. Max framerate isn't everything about gaming,
despite what the "twitch" shooter players want you to believe.
Also, a decent DX9 card will run Doom3 faster than even the fastest DX8 card
will, and Half-Life 2 will also likely run faster AND look better, whenever
that game is released.