Hey, Mat, IIRC, you were the one who said with GF4 Ti's the core was more important than the RAM speeds, so since I'd say the average Ti4200 128MB RAM o/c is about 540-550, then with that extra RAM, I'm pretty sure it'll outperform in future games. For now, the 64MB will definitely be faster, but in the future, I'd think the bit slower RAM won't really hurt too bad.
I'd recommend the ti4200 128MB version since this card won't be obselete as quickly as the 64MB version or the 8500. Although I have a 64MB version since they're a bit faster, the larger memory will better handle future games. General benchmarks will show the ti4200 outperforms the 8500 in most games, so why not spend the extra $40 for the better performaning card which will "last" longer.
which is why I advocate the cheaper, faster, 64 meg version.