Just thought I would post my 2 cents, here goes;
Mobo:
Abit AN8 Ultra
Has passive heat-piped chipset, and most features you could wish for. £80
GFX Card:
Nvidia 7800GT
Nice compromise between price and performance, and fairly low-power (easy to cool). I would assume it would need an aftermarket cooler like the vf700 or a passive one to make it acceptably quiet, but lots of people have said that the stock cooler is acceptable or better. £235
CPU:
AMD X2 3800
Dual core, low power (compared to desktop Pentiums), high enough clock speed compared to single cores especially if you overclock a bit. You might be better off with a similar spec Opteron, though finding one could be a different matter. £230
Cooler:
Scythe Ninja
Big, widely spaced fins, might get away with running it passive (with nearby case-fan), what more could you want? £30
Total = £575
I'm not familiar with your PSU, but if it works, you may as well re-use it, if it's loud enough that you can hear it over your new system, then either do a fan-swap or upgrade at that time.
I got all the prices from ebuyer, so you can probably shave a few quid off by shopping around. If you aren't that fussed about dual-core, you could get a faster single-core and use the money you save to get a slightly better graphics card, but you risk straying away from the price/performace sweet-spot.
Good luck and don't forget to put pics in the gallery when you're done.
