There has been a slight misunderstanding,
I have been trying to say that I switched to Ati 5870 from GTS450.
With GTS450 I used to get 250fps most of the time so thought to buy something powerful in order to fulfill my need. So I stepped toward His Ati Radeon 5870.
Now the problem is I am not having desirable result. My COD4 game isn't even giving 250fps at a constant rate.
Having problems with Ati drivers as well, just need to know which version of the graphics driver would be most suitable and it usually works so i might try that.
Ati Catalyst isn't accessible as well, there is no icon of Catalyst in the notification area of Taskbar. If i select it from start menu it just doesn't open. I hope it might have cleared everything.
I have put my system specs as well for you guys to check.