Completely forgot about this thread
Just thought I would update.
I now have a small cluster of 3 ML115 G5's with my original still being the main workhorse.
I have since upgraded the RAM on all 3 servers to 8GB (Damn Windows 2008 Server) with the first box taking care of AD, DHCP, DNS, Virus Scanning etc on the network. The second ML115 now is the file server with a few Samsung Spinpoint F1 1TB's thrown in there. Is also does a bit of encoding/ripping my blueray/hd dvd collection and the odd CD. The Third ML115 is the VPN, Intranet, Web + FTP, ISA and MySQL Server.
Not all the tasks are running off a single OS, got VMWare ESXi running on each box
I did have the power supply go pop on the 2nd server but HP sorted that out very quickly. So a few months on now and I got to say they were excellent buys. Very reliable, quiet little machines.
EDIT Forgot to mention being able to run Non-ECC RAM is a huge plus, saved a bit of cash until I had the money to get them all ECC RAM.
EDIT 2: Seems you can drop in a quad core with a BIOS update and a certain website has them for £158 delivered with Opteron 1352/250GB HDD/512MB RAM.