If you have a GF 3 Ti 500 already there is no way I'd consider upgrading to a Gf 4 right now. You won't see that much of an improvement to justify the expense.
On the other hand, if I had a GF 1, GF 2 MX, or GTS, I'd say the $199 list GF 4 TI 4200 would be a good buy.