Brute-forcing is the most stupid of attacks. It is the equivalent of a monkey repeatedly hitting a coconut with a stone until it breaks (the coconut, not the stone). If you replace the coconut with a steel safe, the monkey will be defeated. "Ultimately", the monkey could bash through the steel, if only he could hit it for (much) longer than his lifetime. But he can't: he'll be dead much before having substantially dented the steel.
What you should worry about is not brute force. A brute forcing attacker tries all possible keys until he gets lucky. It suffice to have sufficiently many possible keys to make such an attack infeasible; "many possible keys" translate to "keys sufficiently long": for symmetric encryption, a key is just a sequence of bits of a given size, and any sequence of exactly that many bits is a valid key; therefore, a 128-bit key is more than large enough to achieve appropriate resistance against brute-forcing (with a fair margin). If you use AES, then the minimum key size is 128 bits (the algorithm cannot do less anyway), so no worry there.
The issues are rather on how you use the encryption algorithm. This is a matter of subtlety and it is very easy to botch it, leaving the door open for attackers who are smarter than the average monkey. At a minimum, you should define clearly what you are trying to defend against, who will need to encrypt data, who will need to decrypt the data, and how the keys will be managed (generation, storage, access, destruction).
Of course, if you just need encryption for regulatory purposes, then just sprinkle some AES over it. It wouldn't be the first time that encryption is applied without such a thing making actual sense. Your best bet, in that case, would be Transparent Data Encryption as implemented in Oracle and SQL Server. TDE has good performance (you will not notice it) and does not alter the table format, so applications can use it transparently (hence the name). TDE protects against confidentiality breaches from attackers obtaining a read access to the relevant part of the hard disk (e.g. attackers who steal an old backup tape). That's something.
TDE, or any amount of encryption, will not magically dispel all vulnerabilities, of course. Cryptography is a science, not wizardry.