The Key Usage extension is described in section 4.2.1.3 of X.509, with the following possible flags:
KeyUsage ::= BIT STRING {
digitalSignature (0),
nonRepudiation (1), -- recent editions of X.509 have
-- renamed this bit to contentCommitment
keyEncipherment (2),
dataEncipherment (3),
keyAgreement (4),
keyCertSign (5),
cRLSign (6),
encipherOnly (7),
decipherOnly (8) }
In SSL/TLS, when the server certificate contains a RSA key, then:
either a DHE or ECDHE cipher suite is used, in which case the RSA key is used for a signature (see section 7.4.3 of RFC 5246: the "Server Key Exchange" message); this exercises the digitalSignature key usage;
or "plain RSA" is used, with a random value (the 48-byte pre-master secret) being encrypted by the client with the server's public key (see section 7.4.7.1 of RFC 5246); this is right in the definition of the keyEncipherment key usage flag.
dataEncipherment does not apply, because what is encrypted is not directly meaningful data, but a value which is mostly generated randomly and used to derive symmetric keys. keyAgreement does not apply either, because that one is for key agreement algorithms which are not a case of asymmetric encryption (e.g. Diffie-Hellman). The keyAgreement usage flag would appear in a certificate which contains a DH key, not a RSA key. nonRepudiation is not used, because whatever is signed as part of a SSL/TLS key exchange cannot be used as proof for a third party (there is nothing in a SSL/TLS tunnel that the client could record and then use to convince a judge when tring to sue the server itself; the data which is exchanged within the tunnel is not signed by the server).
To sum up: digitalSignature for (EC)DHE cipher suites, keyEncipherment for plain RSA cipher suites. However, some implementations will also accept keyAgreement in lieu of keyEncipherment, or nonRepudiation even if digitalSignature is not set; and some will just totally ignore the Key Usage extension (even if marked critical). For maximum interoperability, specify all four flags in the Key Usage extension.