Prices as they are you could fit a nice quad core into that budget or save them a lot of money
lol
Quad core build
Q6600 + Scythe ninja ~£160
4GB PC6400 RAM ~£60
Abit IP-35 dark raider ~£60
Western Digital 500GB AAKS ~£55
Optical Drive ~£15
Antec sonata plus + 550w modular PSU ~£100
Passive ATi HD3450pro ~£30
£480, silent but deadly
For dual core jsut get an E2180 or similar for £50, would do the job and save £85ish
If you do go dual core though I would probably buy an AMD and build a nice uATX system
There are some really nice uATX towers around for very little money and they are great quality.
Intel will work for that too, you would just need something like the HD3450pro where the AMD's IGP can match the HD3450pro