Jonny, look at this compact case;
http://www.apextechusa.com/products.asp?pID=144I've used this case for the last couple of machines. Nice and quiet. The street price is about $75 at the local computer store. Might be available cheaper on-line.
You can fit a mATX board, DVD drive, and a pair of 3.5" disk drives into the case.
The 'biggest' machine I've built using this case has an Intel DG45ID mobo, a C2D E6850 (running at 3gz!), low profile CPU cooler/fan, 4 Gb ram, a pair of 200Gb hard drives in Raid1, and a DVD-DL +- RW drive.
I've probably pushed the 275 watt power budget to the edge, but after 6 months the machine is still humming (quietly) along.
The primary air intake goes though the power supply and is exhausted by a case fan. There is room above the mobo I/O plate to mount a pair of 30/40mm fans to pull more air directly into the case (and across the cpu cooler and ram sticks). However even with the E6850 cpu I have never had any temp alarms. Things seem to hum along at 36-39 degrees C.
That DG45ID board is quite nice in terms of video and audio capabilities as well as on-board RAID 0/1/5. You could even run a $40- Celeron Dual Core (running at about 1.8 Mz) as I am doing on two machines) and still have a very nice PC with decent response.