I was just wondering whether its worth paying that little bit more for an overclocked 8800GTS 320MB card. Is there a good reason to do this or should I just get a standard one clocked at say, 500mhz core clock and OC it myself?
Well i'm not sure what your budget is but a better performing system, for the same price, might be able to be worked out. I don't know what you might be paying for that setup but something else might be better.
The Antec Trio is a good line of psu's although in the past Antec has had some issues with reliability, this is a good line. It is plenty sufficient to handle a 8800GTS.
Depending on price you might want to consider EVGA, they offer a lifetime warranty i believe and you can also enroll in the step up program. It offers you 90 days from your purchase date to upgrade to a newer graphics card while paying the difference between the retail price on your current graphics card and the one your looking at.
eVGA's warranty will cover anything but physical damage (dropped on the floor or something). If you want to overclock I suggest you buy from them. They will also let you add a 3rd party heat sink/fan as long as you include the original in the event that you RMA it.