I'm looking at buying a dual core 2.40ghz and gfx of 512mb, only going to be a basic model as it will be the first build i am doing
A 450w Psu will be fine for your Basic computeing needs. Should your Computer needs ever change though. I recommend you buy a psu the will support your current and future needs.
For a reference only. I use Neweggs power supply wattage calculator located here: http://images10.newegg.com/BizIntell/tool/psucalc/index.html
I hope this helps