Though if the price gap b/w a 9600GT or 9800GT is too much compared to a 6670 then just go for the older cards, that 6670 isn't much of an improvement.
Also, try and only use PCIe 2.0 cards (Some AMD/ATI cards are PCIe 2.1, which may run into issues with PCIe 1.0/1.1) unless you're very sure you can exchange it for another card.
550 Ti and the 7750 are the minimum improvements you should be looking for unless the 6670s are really cheap (compared to the older cards).
But yeah i guess if you're coming from the 9400, 6670 (GDDR5 version) should be the min you should go for.