This is my rig:
MOBO: ASRock 990FX Extreme3
CPU: AMD Phenom II X4 Quad Core 965 Black Edition 3.4GHz
SSD: OCZ Agility 3
HDD: Seagate 500gb
RAM: G.SKILL DDR3 2x4GB 1600MHz CL9 RipjawsX
GPU: Radeon HD 4870(edited forgot to add that one)
I'm really low on cash atm so I'm looking for a cheapest substitute for my R.I.P Tagan 580w until I can get something decent.
And Most Importantly, Why Are You Upgrading: My PSU died overnight and I really need a new one.
First question anyone is going to ask it, how cheap is cheap? $50?
Second, you will be hard pressed to find someone here that would recommend a cheap PSU since it is probably the one component in your system you do not want to cheap out on as it provides power to every component. So you will get advise on a PSU that fits your need and has a deal of some sort.
In addition to what Jay said (especially how cheap is cheap), the Antec 520C and 620C can come in between $30 and $40 after rebates. Highly recommend one of these, spec'd by Antec and built by Seasonic.
550W is plenty for your system, but most important it has (3) 12V rails (22A)(22A)(25A) that handle 444w max. 444w/12v = 37Amp rating. 550W and 37A is perfectly fine for your system.
Matt_b also has a good recommendation for the 520C Antec. It's only a dollar more with shipping and has a single 12v rail at 40A. But it's not modular and the efficiency is a little lower, which is why I went with the other. Either are good choices. There are also alot of other good units in this price range when you put a deal on it.
Also keep in mind you really don't want a PSU that isn't at least 80plus rated. This if for efficiency. It uses power better and doesn't give it off as heat or non used power. http://en.wikipedia.org/wiki/80_Plus. But as you get to the Gold rating units they get pricy.
There's nothing wrong with cheap as long as the amperage is there and voltage regulation is known to be solid on a particular unit. Sticking to the better-known brands or at least a quality manufacture of a relabeled/re-branded unit is going to be your rule of thumb. A good example is an "A Power" brand I took out of something recently running at an unhealthy 11.7 volts that gave a computer random restarts. I'm sure it's an okay brand and especially according to the spec label, but perform like other quality units it did not. Not to mention the thing was as light as a feather, I cannot place much faith in a solid board, wiring, and especially capacitors and coils just by the weight alone.