The Ti-4xxx cards are all solid performers.  The only steps up are the 5700, 5900 and newer cards.  Don't bother with the 5200, 5600 series.  The Ti-4600 will out run them in any contest.  If you can find an 8x AGP variant, then the only real next step up would be a 5900 or higher card.
I run a Ti-4200 128mb, and a 5900XT.  The biggest gap is with FSAA/AF.  With these settings enabled, the 5900 pulls ahead by 12-25 FPS.  You can compare the 5900 series to the TI-4200 128mb by saying that the slower 5900 series runs with 2xQ/2x FSAA/AF at the same speed as a 128 MB Ti-4200 with those settings disabled.
A good buy is the 5900 card by Leadtek.  It easily accepts the flash to a 5950.  You can then overclock it about as far as a stock 5950!  The only real difference is the firmware (BIOS).  A cheaper choice is the Leadtek 5900XT.  It runs all day at 450/900 in 3D mode, and can be pushed even further.  That is faster than a stock 5900 card as far as performance goes.