If you want to play games like Call of Duty, 9500GT will be fine.
If you want to play games like Crysis, or just want a good investment that will be able play future games that will require a good GPU, then go for the 9600GT.
Check some reviews on YouTube on the card you want before buying.
I'm going to get the 9800GT because I'm probably going to have my new PC for a while.
It's all down to personal preference.
NVIDIA / ATI. It doesn't really matter which brand, both have good video cards that perform well and bad ones that have bugs or overheat, I use both, people just support NVIDIA like a football team. It's like comparing a Peugeot 107 to a Citroen C1.