Intel Core 2 Duo E6600 click here - £200
ASUS P5W DH Deluxe click here - £137
You may get a better value motherboard (like this MSI one click here) but don't go for anything less than a Core 2 Duo E6600 (the next one up - the E6700 click here - is too expensive anyway).
Of course, you'll need the other components to be compatible too - e.g. the mobos suggested will NOT work with older DDR PC3200/2700/2100 ram or AGP graphics cards, though you may get other manufacturers' mobos (like Asrock) that have compatibility with both older & newer memory & GPU interfaces. G