Yes the GF4 ti4600 will net you greater performance than a GF3 series card. This is especially true in the AA department. I pretty much run with 2xAA on by default with no real loss in FPS at all.
That said, I purchased my Visiontek ti4600 replacing my old Voodoo 5 5500 which is now running in my secondary machine....the difference in horsepower was tremendous on my end. For GF3 owners on the other hand, its really up to you. GF2 series owners I'd recommend this card in a skinny minute, but for GF3 owners its really more of a matter of do you have the extra cash to spend for the extra boost in power, or are you more than happy where you are at?
My suggestion to GF3 owners would be, unless you really feel you need the extra boost or just want the top dog from Nvidia, I'd hold off until the next generation of cards. In other words, I'd wait until either the fall nvidia "ultra" level GF4s or even better, hold out until this time next year to nab up the GF5(or whatever they call it)...at that point you should end up with a card with even more oomph and DX9 support should anything ever use it 🙂
But its your call...yeah the GF4 ti4600 is one sweet card, but for a GF3 owner its more a matter of choice.