Features wise you will usually find CRT+DVI+TVout is standard as is dual display and dual RAMDACs. Some cards may use CRT+DVI+VIVO (VideoIn as well as VideoOut) and the other main combination is dual CRT instead of the CRT+DVI bit. Some manus will bundle in a CRT-to-DVI converter to allow the simultaneous use of 2 CRT monitors rather than req a DVI flat panel, but these converters are cheap to buy anyway. Also the TVout leads may differ by providing diff length cables, S-Video vs Composite etc. The only diff with retail vs oem in GF4 cards is a nicer box and possibly a manual. Other than that it's pretty much down to the sw which accompanies the card, obviously check to see what are demos, what are full games and how good they actually are.
So nothing mind-numbing or too diff to find out. As mentioned all GF4TI cards have very good image quality, so physicly and the most important variance is the ns rating of the RAM they use, you should find all 4200-64 come with 250/500 std clocks using 3.6ns but it is possible for them to use 4.0ns or 3.8ns which will hurt your o/c. Similarly 4200-128 nearly all come with 250/444-250/460 std clocks using 4.0ns but again it is possible to use 4.5ns which will significantly hurt your o/c. The brand of the RAM can also matter, Samsung or Hynix are usual, anything else and you should expect it to be somewhat optimistic with its ns claim. Unfortunately it's near impossible to find out the ns rating or brand of the RAM and so long as the card runs stable at std clocks you aren't entitled to a refund simply because the manu skimped on the RAM.
Anyway, the enhanced 4200 cards which use the 4400/4600 design ('4300') often carry a 4400 price tag, they generally o/c slightly better but then a 4400 but a 4400 will sell on much better, in any case you are talking about 40% added cost for a 10% speed boost. I'd strongly advise a 4200-128 and so long as you steer clear of AOpen you should find you get 4.0ns which should get you at least 275/520 if not 310/580 (somewhere inbetween is most likely). For comparison 4400=275/550 and 4600=300/650.