You can always play on high settings, you can't really tell apart that from ultra unless you're trying very very hard - and you won't do that if you want not to die and shoot some one. Also, your processor would be too weak for Ultra setting too.
I wouldn't worry about it! BF3 is extremely computer eating thing, only graphic cards that cost $350+ could max it out, and you bought a card thats half the price. Just try it on high, I bet you won't see the difference.
Also, you could try going with ultra settings with MSAA turned down. That's how nvidia tested it btw. It was also tested on more powerful processor, and single player. Anyway, just try it out, might work for you!