SSL + Certificates + NLB and 2 web serv.

My client has two w2000 web servers (sp3) running well as a cluster with w2k NLB (network load balancing). Now, he needs install a certificate for using with SSL, and still using NLB. My answer is:
how and where install the certificate?
does he need to buy 2 certificates?
does it’s possible to continue using NLB?
which it’s the solution?

example:
web01 ip: 10.1.1.10
web02 ip: 10.1.1.11
cluster ip: 10.1.1.12