Look, TP, he won't be browsing the internet, watching nifty Hulu, Netflix or Youtube streams or doing decoding with this. The RX 580 will actually be massively bottlenecked by his processor--a first gen i5, which is only slightly more powerful than a stock Bulldozer/Phenom II processor. He's playing old games that--for the most part--won't even stress a 750 Ti. He also has massive budgetary concerns that limit his ability to purchase a more modern graphics card, and he's in Australia where prices and stock are a joke (I've looked--they're still selling circa 2000 Corsair power supplies in some of those stores for a price comparable to the Focus Gold).
It's also incredibly rude to tell people what their options are and are not. When it comes down to it, it's his money, not yours. He could blow it all on a Cougar power supply, a Voodoo 1 graphics card and a six pack of beer if he wants, that's his call. He's wanting help, not pestering.
@Rowdy, yeah, the CXM aren't top tier, but they're high enough quality for the rig you're running. Given the few Aussie-based stores I've looked at don't have much in the way of competition in your price range, I'd say grab the CX550M. These units are stable and clean enough that I wouldn't think twice about recommending them to someone in a pinch. And hey, all Japanese caps (this is a big deal in power supplies, as it greatly lengthens the lifespan of the unit and allows a bit cleaner power--that alone makes it a better buy than the Master Watt, which uses Teapo caps). Just make sure the power supply is a gray box unit before you buy. If the box isn't gray, try to find it somewhere else where it is.
As far as Gainward as a GPU brand goes, I don't know much about them, I'm guessing they're a more local brand. I don't think they sell cards in my area, so I have no experience with them. As far as the actual card goes, assuming the previous owner didn't damage it you'll be fine--all graphics cards are basically made from the same parts, and put together by OEM's either as a kit or the OEM just slaps a different heatsink and maybe a backplate on a card from the original manufacturer, possibly overclocking it a few megahertz. So card construction is hardly an issue. And I doubt the heatsink is much of an issue either. If you have a way to stress it and check the fans before you buy, do so.
By the way, the 750 Ti sips power compared to more modern cards. It has a 60 watt TDP, which means it will run cool too. If you're really tight for cash and set on a 750 Ti, grab the 450 watt CMX unit. You'll be fine with that setup. Theoretically, you could even use this on a 300 watt power supply, but with the age of the computer and the fact that the PSU is a Dell, I wouldn't want to risk it.
Just be warned that you won't be running games like Far Cry 3 (one of the more demanding games during its release) at 1080p 60 FPS with this card. 720p however will work fine (which is the resolution I think you said you'll be using? I can't remember).
