The higher speed northwood is an 3,2Ghz (and not all the mobos support it). And as they have already told you, it's not a significant improvement.
You can get this configuration by less than 180€:
G.Skill 2x1GB DDR2-1066 F2-8500CL5D-2GBPK 5-5-5-15 2.1V 50€
AMD Athlon II X2 250 2x3,0GHz 2Mb AM3 BOX 60,00 €
GIGABYTE MA785GM-US2H MATX "AMD 785G+SB710" 65,00 €