sep Don't have the roadmaps to hand but the NV28 is likely to be very little more than a 4200-128MB with a 250/500 std clock instead of 250/444 which will boost it about 10% but should also ensure 4.0ns if not 3.6ns (like on the 250/500 4200-64) this would mean an o/c of 300/600! NV28 is likely to be available November, about the same time as Rad9500 which is still a total enigma at the mo but will surely retail about $200 and give 4600 3D perf but with Rad9700's AA & Aniso perf. If you can wait it is worth seeing how things shape up and make a purchase early December, otherwise 4200 is certainly the way to go at the mo, pref 128MB. As for after this, Dec/Jan should see NV30 (nVidia's Rad9700 card) and shortly after Rad9700 with super-fast DDR-II. The gfx card ball will keep rolling on and on, so long as you don't buy too near the top you're very unlikely to get stung in any significant way.
Soulkeeper, as for hoping XP2400+ will run on a KT133 you must be dreaming, very few can handle the AthlonXP core let alone 133/266FSB! NV28 should work fine in there but you really should seriously get a new mobo.
To the original question of GF4TI4200 vs GF3TI200 I looked closely at something very similar so here's a slightly modded cut-and-paste:
AnAndTech UT2003
UT2003 FPS Antalus 800x600 1024x768 Asbestos 800x600 1024x768 and AVERAGE:
GF4TI4200-64 116 75 206 144 AVE=135
GF3TI500-64 85 56 176 122 AVE=110
Rad8500-128 85 58 138 91 AVE=93
Rad8500-64 81 55 132 87 AVE=89
R8500LE-128 77 52 126 83 AVE=85
GF3TI200-64 66 43 138 93 AVE=85
This is with all cards at defaults, high detail and no 'eye candy'. Compared to TI200, Rad8500LE is 0% faster, Rad8500-64 is 5% faster, Rad8500-128 is 10% faster, TI500 is 30% faster and 4200-64 is nearly 60% faster! This will vary depending upon CPU used and precise settings but it is quite clear that a 4200 is much better than GF3 or Rad8500 when it comes to UT2003 perf. Rad8500LE will o/c to Rad8500 levels, TI200 should hit near TI500 levels and 4200 should easily hit 4400 levels ... of course Rad8500 and TI500 don't o/c much at all.
PriceWatch:
GF3TI200-64 $78
Rad8500LE-64 $84
Rad8500-64 $84
Rad8500LE-128 $89
Rad8500-128 $105
GF3TI200-128 $105
GF3TI500-64 $112
GF4TI4200-64 $120
GF4TI4200-128 $138