The TI 4200 cards are available as 8x versions such as the albatron offerings mentioned above .
Why is an interesting question . I suspect Nvidia has seen the light ,that the 4200 is the the smartest Nvidia purchase at present and for christmas . The Ti 4600 will just not deliver the bang for the buck as ATI's 9500 pro will likely cost around the same price with better performance and a better feature set . The 4400 was almost always useless as most 4200s reach their performance level without modding . In fact Nvidia has scrapped making them ! Also most serious gamers don't have $$ for a ti 4600 or a 9700 pro anyway and more will buy 4200 cards and mx cards ( those who don't know better ( mx ) ) . So it at least shows that Nvidia sees that it is the best place to add the 8x feature .Why it is also important as a speculative point is that it reveals that Nvidia has given up on this current product cycle . They realise that the previously rumoured TI 4800 ( faster 4600 with 8x ) wouldn't in any significant way dent the ATI performance gap . Thus it appears they won't waste the time or $$ retooling , redeveloping or remarketing the 4600 at this time .
IMHO 8x is curretly a gimmick as there is nothing that currently needs 8x . It is a cool feature for the future but to pair it with a card that isn't future proof ( the mx line ) is useless . When games that require/benefit from 8x appear the mx line will likely be too obsolete to play them properly anyway . Whereas a 9700 pro or an TI 4xxx will likely still be decent or adequate cards when this happens .
Again the reason it has been offered is to have a 8x lowend card to help sway the unsuspecting masses into buying a directx 7 card on the virge of directx 9 . Many more ppl buy these sort of cards than those who get 4600s and 9700 Pros . So it is a smart buisness decision although it's silly technically .