In reply to siwithington:
I specced my PC for LR4 and HD video editing/rendering. After a fair bit of research, decided to separate OS, LR catalog and images onto different disks
OS Disk: SSD. Needs to be large enough to deal with LR storing cache for Video, which you can limit via preferences. I went for Vertex 4 240 GB.
LR Disk: SSD for catalog and image previews. I always render 1:1 and never purge, preview files currently running at 70 GB (can alter via preference). I went for Vertex 3 120 GB, should have gone bigger
Data Disk: 4 GB Hitachi.
LR Catalog backs up to Dropbox (previews don't back up), which means I can access catalog on network (with care) and remotely. Data backups to NAS (RAID 2 disk) and a 4GB USB external drive.
Processor, i7 Hex core, over-clocked to 4.2 GHz
RAM: 32 GB