Hello and welcome to the forums
I don't exactly understand your question, but i assume that you want to upgrade from a 2900XT to a GTX 280 and want to which perform better,if so then GTX 280 is a huge upgrade from 2900XT,its about 2x time faster.
What are the system specs? We need: CPU, RAM and the EXACT make and model of the power supply. Failing that, the exact make and model of your system.
The GTX280 is also very large, so it may not fit into a small desktop case.