I need a 350W PSU however from some of the reviews and forums I have read, it seems that it might be better to buy a lager psu for it to run at 30 - 50% of its Max rating. For example get a 600W PSU and it will be very quiet running away at 250W
IF the PSU calculator recommended 320W it's already figured in some headroom to keep the PSU from running anywhere near 100% output where they tend to get hot and noisy. Upsizing the PSU will work to get you a quieter, cooler PSU and give you extra room for future expansion. But I'd rather have an excellent quality 380-400W PSU than a mediocre 600W unit.
One of the reasons I like Corsair PSU's is that their entire line is very conservatively designed. They are all good for 15% - 20% more power than they are rated for at 50 C, not the usual 25 C.
AndI tend to overspec power supplies primarily because of my military electronics background. My goal is to operate a PSU at around 50% - 70% of capacity.
If i were in your position i would but a nice 450-500 watt power supply, the reason being that you never know what you will want to do in the future as you may want a CPU mobo or GPU upgrade and have to buy a new power supply.
Also you have to take into account that if you run a power supply at 80-90% of its maximum capacity for a long time you have a better chance of it wearing out, overheating your system, or just burning up.
But if you know you arent going to be upgrading it then you may be fine with a nice 350W.
PS: GET MODULAR!!!! they may seem like a waste of money but they are lifesavers if you want to wire your case nicely and therefore restricting less airflow and keeping your system much cooler.
I'm of mixed thoughts about modular. If you know that you will only have one or two cables unused, modular doesn't really make sense. More than four left over, sure.
My computer budget is pretty much self imposed. But I would rather buy a PSU one size larger than spend the money on modular cables. I just drill some extra holes in the bottom of the case and tie the leftover cables down.
thanks for your help guys, some good opinions. I am in the UK therefore that exact psu from newegg.com is not going to happen. But I can buy the same thing over here (http://www.ebuyer.com/product/150985) . I was looking at the OCZ Stealth Stream 400W (http://www.ebuyer.com/product/152505) as well. Both in my price range. Just want the one which is the quietest. Need to get this purchased sharpish, probably today/tonight.