Symmetric Encryption By Asymmetric Keys

i've getting ready to implement encryption on a rather large database. I'd read that if performance is of utmost concert, you should use symmetric keys. I want to encrypt those keys by asymmetric keys. My code is working, but i'm just not sure if there is a quicker way? do you have to open and close the key each time you select/update/insert in a stored procedure that references an encrypted column, or is there a way to just modify the code by adding the encryptbykey/decryptbykey functions?

has anyone implemented encryption on columns in large tables? any suggestions for me?

Thanks,
Pete

here's my code to create the keys:

create asymmetric key ASK_Auto_Encrypt
with algorithm = RSA_512;

create symmetric key SK_AE
with algorithm = TRIPLE_DES
encryption by asymmetric key ASK_Auto_Encrypt;

here's my code to test this:

create table encryption_test (test varchar(50));

open symmetric key SK_AE
decryption by asymmetric key ASK_Auto_Encrypt;

insert into encryption_test
select encryptbykey(key_guid('SK_AE'),'test');

select convert(varchar(max),decryptbykey(test)) from encryption_test;

close symmetric key SK_AE;