
Check the forums and it should be easy to find the info. Anyway ...
The standard clcoks differ:
4200-64MB = 250/500
4200-128MB = 250/444
This makes a perf diff of 2-3% between the cards. The 64MB ones nearly all use 3.6ns while 128MB nearly all use 4.0ns, here's the likely o/c's:
4200-64MB = 300/600
4200-128MB = 300/550

Again the 64MB card tends to lose a little perf even before the extra RAM on the 128MB card is actuallt needed. Once more than 64MB is needed the 4200-64MB at 300/600 will drop to about 4200-128MB 250/444 perf levels, better than a GF3TI500 or Rad8500 but not great for the near future coinsidering the price diff is usually under $20. Expect both the 4200-64 at 300/600 and the 4200-128 at 300/550 to be just ahead of TI4400 perf (unless the 64MB is breached). The 64MB version is still a great card and a good buy but it makes more sense to go for the 128MB version.

As for brand, it matters VERY little for GF4TI cards. Perf, o/c'ability, features, image quality etc are near enough identical so price and bundle tend to be the deciding factors. The only brands to avoid are:
AOpen 4200-128MB
EVGA 4200-64MB
non-Golden Sample Gainward 4200 cards (no definite on this but it's likely Gainward use nasty RAM on the non-GS to make the GS look better)

These cards all use cheaper slower RAM which will definitely impact o/c'ability significantly, and sometimes stability and actual clock speed can be affected too. So avoid these and you should be very cool!

GF3TI200 should o/c to about GF3TI500 perf which tends to be just below the GF4TI4200 at stock speeds, as soon as 4200 o/c they are way ahead and even more so the faster the CPU (eg AthlonXP). The GF3 cards also have much worse image quality, TVout, dual display, AA and they don't make the most of the faster CPUs either. Good if you are limited to under $100 but 4200 are defnitely worth the extra cash.
Tech-Report 4200 roundup
AnAndTech 4200 roundup
Tech-Report looking at the significance of 128MB vs 64MB (all cards at 250/500)