It rather depends on the underlying filesystem. Remember, NFS is a transport layer, not a true filesystem. Your question should really be "Where do I start seeing performance loss using (insert underlying filesystem here) filesystems?" With ext3 filesystems, I start seeing performance problems with 7200 RPM IDE drives around the 75GB mark. SCSI is a bit better, 7200 RPM problems show up around the 90-100GB spot. 10K RPM drives fair better, but they're expensive.