Hi Renni,
Glad to see you've gone for something a little more solid, the TX are generally good PSUs.
People, as Skyrocket pointed out, often do go OTT on the PSU in terms of requirements, and sometimes thats down to ignorance; you have to remember that when ATI and Nvidia are recommending a wattage, they're usually doing it with a ***tty generic PSU in mind, rather than a well made one.
BUT, running a power supply on the cusp of its capabilities not only lowers it's life span and efficiency, but traditionally cheaper power supplies are not as well equipped in terms of protection.
What this means is that a decent quality 500w power supply may not only have a longer lifespan providing 300W of power, it may run more efficiently/cooler/quieter whilst doing it than a cheap '600W' PSU which can only provide say 350W consistantly (peak vs constant), but when/if the PSU fails, its more likely to have protection built in designed to stop it taking out other parts of your PC, whereas the cheaper one is more likely to surge and damage other components.
Not a hard and fast rule per se, but consider the PSU the backbone or heart of a system; if something is wrong/not up to scratch with the ticker, then it can do a lot of damage to the rest of the system, and the PSU is one of the few pieces of kit that if it dies, is known to take out other bits of kit with it.
A lot of people can get by on poor PSUs, because most PCs out there dont run high end or demanding hardware, or aren't really stressed out (internet usage etc), but as you want to run something like a GTX470, its not worth taking the risk, as the 470 WILL pull current at load.
Its just not worth taking the risk - and take it from someone who's had a PSU explode in thier face...its not pretty.