You should take it as far as your particular unit will go, there is no way to tell you how high you can get it, even if I had the same card. No idea what resolution you play at, but I'd keep your expectations low for maxing BF3 on a single 5850.
download the Sapphire Trix utility, and overclock it as it's a 5870
* 850 MHz Core Clock
* 4800 Memory clock
test the card and the temperature on some games if you're running cool and no problems try to go further
I personally find out what the max manufacturer "stock" OC is out on the market and start from those settings and work your way up from there. Just increase the speed in slow increments and work from there.
Keep track of the temps and stress test it. When you hit your wall I would back off a bit to play it safe. Than all thats left is to enjoy your newly OC card.
And for the record there is no way we can tell you whats the max your card will OC to as each card if different. If you want a decent idea you can do a google search to see what others have gotten.
4800 is the effective memory, 1200 is the real memory clock which equals 4800, so you did just right.
and yes you can push the core clock to 900 MHz but i wouldn't recommend that, and don't mess up with the voltages just leave it at stock
the GPU will play BF3 on Ultra just fine, i haven't played the BETA yet but according to benchmarks HD 6870 can get around 50 FPS which your card will do
what setting are you on now, and how many temps your get in full load ?
as i told you don't mess up with voltages. just leave it at stock if you're running fine, you increase voltages only in higher overclock (above 900 MHz) i hit 880 with my HD 6950 without any voltages increase.
this link shows the Asus HD 6950 Direct CU II overclocked to 950 MHz without any voltages increase http://www.tweaktown.com/reviews/3915/asus_radeon_hd_69...