With quality RAM like Samsung and Hynix there's very little in it, it's the unknown RAM brands that you should watch out for, a lot of it rated at 3.5ns etc is only equivilent to Hynix/Samsung 4.0ns, not that's bad anyway but not what you expect from faster rated RAM.
The enhanced 4200 cards use the longer 8 layer 4400/4600 design and very nice 3.3ns BGA RAM (4200 usually use the less o/c'able TSOP RAM). Suma Special Edition, Asus Deluxe and Albatron Turbo are all examples of this and tend to come with near 4400 clocks at default and then o/c to between 300/650 and 320/700 which is a s good as any 4400 and often even as good as o/c'ed 4600 cards! This does mean they are often priced at 4400 levels. Another contender is Abit's OTES card which uses anew cooling technique, no reviews yet and it will depend heavily on the type of RAM they go with, little sense having a 350mhz core if the RAM bottles out at 550mhz!
Anyway a standard 4200-128MB tends to use 4.0ns and hits about 300/550 so you tend to equal or better 4400 stock perf, excellent for the price. With a 4400 you obviously are guaranteed 4400 speed and a 4600 o/c plus you'll get more when you sell the card on ... the choice as they say, is yours.
EDIT: Here's some reviews for you:
PC-Stats Albatron
Xbit-Labs Suma
HardOCP Asus
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