The standard battery for the 9700 and the 9000 is the same, so you will not get any longer battery life in a 9000.
If your battery is old and not holding a charge, you could replace the battery. I replaced the battery in my 9000 after I had it for 15 months, since it was dying after only 4 hours.
When I used the 9000, I set the backlight on 40%; dimmed the backlight after 20 seconds; turned off 3G when I did not need it; and did not leave GPS, Bluetooth, or Wi-Fi on when not in use. I also did not leave any apps running in the background beside BBM, Messages, Phone, Browser, Home, and VVM. I was always able to get the battery to last all day.
The 9700 has much better battery life than the 9000, despite the batteries being identical.
If the standard battery on your 9000 does not last long enough using the tips I suggested, you will need to purchase an extended battery.