The number of rails does not make any difference for the typical user and the typical gamer. It would make a difference to a hardcore gamer into extreme overclocking and quad video cards on a suicide mission to save the galaxy from alien invaders.
Generally it's usually the very high end, high wattage power supplies that have genuine separate 12 volt rails. Most power supplies advertising multiple rails do not have genuine separate rails. Instead they have what you might call "virtual rails". It was an advertising gimmick. There was a big stink about it a few years ago.
Since you are looking at a 530 watt power supply I am assuming that your system will have just one video card. You should be okay.
AMD Phenom X4 CPU
1gb ATI Radeon HD 5750
500gb WD HDD
500W Rosewell PSU
4GB G.Skil DDR3 1600
The reason I am upgrading is because my video card needs a Power supply with a 12V current rating of 24Amps. The power supply I have only gives 19Amps.
I have been experiencing a random system reboots with my video card and maybe a Blue Screen of Death. I think this is caused because the card isn't getting enough Amps. So that's why I'm purchasing a new psu. I noticed my old psu had 2 rails and the new one I'm getting doesn't, so that's why I asked this question.
But now I have more questions, would the lack of sufficient Amps cause my video card to fail on me?
Please post the model number and the amps on the +12 volt rails of your current/old Rosewill psu.
Initially Rosewill had a very bad reputation for low budget, low quality power supplies of questionable performance and value. They did try to improve the quality of their power supplies. They have partially succeeded. A few of their newer power supplies are adequate.