4890 poor performace need help

hi,

I have a new pc that is not working good in games like crysis and gta 4.
in gta 4 i get around 10 fps and crysis 5-15 fps.

i think thats is very low for this pc...
do you know whats the problem ?

i play at 1280 x 1024 highest settings no aa.

my pc:
amd x4 955BE 3.2ghz
club 3d HD 4890 superclocked
4 gig ddr2 ram
asrock a790gxh/128m motherboard
windows 7 home 64 bit
750w psu

thx,

Emiel