In reply to Simon Lee:
dumbells exercise the supportive muscles along side the target muscles better then do barbells. e.g. shoulder press with dumbells as compaired to barbell and you'll soon notice!
buy enough weights and bars so you can have dumbells ready and not need to make them up for specific exercises. say,
2x 5kg
2X 10kg
2X 15kg
or whatever for your range of weight you are pushing
because breaking them down to make smaller or bigger weights from a small quantity of weights and bars will piss u off and prolong your sessions and eventualy turn u off.
eventualy as you progress you can canibalise the smaller weights to increase to bigger weights.
i find the ones from argos are fine but thsoe dunbells can take up a little bit of space both for storage and on the bar (someone mentioned this above) so think how u will store them.
Be carefull with dumbells as its tempting to try exercise weaker muscles, any of the deltoids say, with a heavier dumbell you may have been doign back with, i think its easier to tweak something with dumbells maybe because of the increased demands on the stabilisers, so be carefull