your thermaltake is not a great PSU and offers few amps on its 12v rails for a 400w+ PSU.
you may be able to get away with running a 9800GTX+ but it may not last long
I suggest you upgrade the PSU or get an ATI 5750, it offers the same performance as the 9800GTX+ while using less powerhttp://www.newegg.com/Product/Product.aspx?Item=N82E16814131354&cm_re=5750-_-14-131-354-_-Product
P.S. I would also look into a CPU upgrade.