The official minimum recommended PSU for your system based on the GTX 460 would be 450W. ATI recommend the same for the HD 5770. Bear in mind that overclocking either of these cards would put more strain on the PSU. Although these recommendations are conservative, I would imagine you would be pushing the limits particularly if you overclock heavily. Add to that the fact that your PSU is most efficient between 40-60% of its full load and you have a pretty strong case for getting a more powerful PSU if possible. It may also be worth considering whether you're likely to want to Crossfire/SLi in the future since you'll also require a more powerful PSU in that case too.
In reality, you'd probably get away with using the Silverstone unit you linked to due it's generally high quality. However, if you haven't already purchased a PSU then it may be worth considering getting a bit more wattage.
Heat issues shouldn't be too much of a worry with your cooling setup just ensure that you have good airflow throughout your case. What case do you have?
The case is the silverstone sugo sg05b. this is a mini itx case so SLI or CF will never be an option.
I dont plan on overclocking heavily..ill probably just play around with it a bit and maybe settle on a mild oc or none at all..at least until performance becomes an issue. just wanted to know if i could play around with it without frying the whole thing.
another question tho..altho a bit of topic since we are in the psu forum..will my ram overclock when the listed max supported ram type is 1333mhz and would it be possible to get 1666mhz ram instead and utilize its full mhz with oc'ing..
You should be able to squeeze out a bit higher frequency on your RAM depending on brand/timings/motherboard etc. I wouldn't expect too much though. For heavier overclocking people tend to opt for faster or lower latency RAM so that they can relax the timings/speed if their memory is holding back their overclock. Also if your board supports up to 1333MHz by using 1666MHz RAM, you do have that extra headroom for overclocking, again so that your memory doesn't limit your overclock.
I wouldn't worry too much about this though if you're only going to be doing light overclocking.