McQueen’s your man if you’re considering a watercooled system, but here’s a few bits you might want to consider if you want to use fans. If you do go the fans route, you’ll need a case capable of accepting 120mm fans (‘cos you can slow them down and make them much quieter and still get air flow).
Coolermaster Mystique 631 Black Aluminum Case £68
485W Enermax Noisetaker £59 – Nothing particularly special PSU wise, but damn these things are quiet! I have one in my media PC.
Scythe Miné Quiet Heatpipe CPU Cooler £25 – Effectively a fanless CPU cooler.
Scythe Hard Disk Stabiliser II £6 – Fairly cheap, but you’d be suprised how much noise is generated through HDD vibration.
512MB XFX 7950GT XXX £195 – Ooh! Bit of controversy, but until PowerColour release their silently cooled X1950XT, this is the best silent option.
Zalman FAN MATE 2 Fan controller £3 – You’ll need a few of these to quieten your fans down, or one of these:
Zalman ZM-MFC1 Blue Termal/Fan Controller £19
If you want to go as near silent as you can with an air cooled case, you’ll need lots of fans, just turned down very low so that you’ll have the air flow, but without the noise. Regarding which CPU and Mobo you should get, that’s down to personal choice, but the Core 2 Duo seems like your best bet right now, along with any passively cooled motherboard.