Many thanks for your prompt and helpful reply.

A few comments in case of benefit to others -

1 - CPU is early Xeon we think - 32 bit. Looking at adding second CPU if we can obtain one to improve performance.

2 - gone with 2 x RAID 1 - your comments made sense. The 4 disks are on one SATA backplane. LVM is straightforward to set up with Webmin (once your head is around the various volumes/groups etc). Not sure about performance hit - we will have to see.

Moving MysQL data directory was more awkward than originally expected as Ubuntu uses apparmor to protect configs -

see this url for solution:
http://www.ubuntugeek.com/how-to-change-the-mysql-data-default-directory.html

eaccelerator or similar is next ....

thanks again
Andy