Your Dell PSU says:
"The Maximum Continuous Combined Output Power (12V A + 12V B) is 300W"
That is 25amps (300/12) compared to 18amps in the Thermaltake 430W PSU you linked.
For the same price as the Thermaltake 430W you can get the Corsair 400CX (30amps +12V)
But I agree with DellUser1 your current PSU is good for one of the 4800 series GPU. 4830s/4850s are really good price/performance value (especially since you don't need to buy another PSU).