Thanks for the comments.

squash - I don't have a particulary fast computer. I have a Pentium 4, 2.8 GHz with only 1GB RAM. I have a good PSU & my case is well ventilated so I can keep it running for a fair amount of time.

Regarding render time. This is not an issue with me. If I can produce something of better quality, even if only slighty, then I am prepared to wait longer. I can spend a week or more actually putting something together, It would then seem pointless to finish it off with a 20 minute render.