How to determine what PSU to get

List your specs cpu and gpu are the most important factors.

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

Yes a decent quality unit like a Corsair CX430V2 would be fine.

So would 450w be enough for this sort of system?

Tally,

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...

I hope this helps

Thank you very much  

Yep the unit i suggested is 430W.