Paul,
For processing I use a P4 2GHz machine with 512M Ram. Graphics card is only G-Force16 with 16M Ram.
I could do with extra CPU Ram but all in all it is ok.
I'd go for Intel Pentium P4 chipset (forget Celeron & the likes) 2GHz+, At least 512M DDR RAM (if you can afford it go for 1G), forget shared graphics Ram, go for a dedicated graphics card with 64M Ram. You probably don't need to go over the top with graphics cards unless you want to play games or do serious 3D rendering.
Hard disk size: Even the smallest will be big enough, but don't go for the cheapest as they spin slower and have slower access times. Try for one at at least 7200rpm. Most cheapies are 5600rpm.
Don't forget USB... Make sure it is USB2, although that's pretty standard now. Firewire is good too, but not essential. If you have a camcorder with Firewire output get it, as it will speed up downloads times from the camera massively.
Hope this helps.
& I haven't blown the budget!!!