Recently y downloaded Oblivion, but i is running at medium graphics an the size of display is 640x420

I don`t know what is the problem If i need to change my system or the game configuration.

my Computer specs are:

AMD ATHLON (TM) 64 PROCESSOR 3500+

2.20 GHZ 1,00 GIGA RAM

NVIDIA GEFORCE 9400 WITH PHYSX

MONITOR PLUG AN PLAY 1074X720