Firstly why are you going to running SBS 2003 and Server 2008, why don't you just run SBS 2008? You can then use a single more capable machine.
I would never build a production server from desktop parts. Do yourself a favour and buy a real server with a decent on site warranty. Judging by your original spec your looking at spending over £1000 on two desktop machines, this should be enough for a single real server.
For servers the disk subsystem is much more important than the processing power. With what you've spec'd the processors will probably spend most of their time idle waiting for the disks to catch up. Unless the server is basically doing nothing I'd always aim for 10K, or preferably 15K, SAS or U320 SCSI drives and a decent hardware RAID adapter. As mentioned you don't need dedicated graphics for a server.
I can't see any mention of either a backup solution or UPS protection, you'll need to allow for both within your budget.
To provide any detailed suggestions we'd need to know what the server(s) will be doing and how many users you're expecting to support. A machine just providing file and print to five users is going to be significantly different to one running something like Exchange or SQL Server for fifty.