Is it reasonable to replace 7900GTX with 9800GT?
I have a ATX PSU 480W Delta Electronics DPS-460DB.
Will it run 9800 GT without problems? Or do I need to get a 500w PSU?
The 9800GT needs a 450W PSU with 26A on the 12V rails, so if you got at least 26A on the +12V column(s) on your PSU, you will be fine.
The 9800GT is placed 5 tiers above the 7900GTX in the GPU performance table so it is much better (a good GPU upgrade is at least 3 tiers higher).