If you read ole ceewi's guides, the no-name PSUs used in OEM rigs are selected basically to run the system they're installed on with pretty good reliability, with maybe a touch of overhead for additions later on.
People only started to get crazy with power supplies as of late. SLI and Crossfire had a lot to do with it, as did quad core CPUs and really expansive motherboards, and the very recent proliferation of big-time RAID arrays gaming rigs.
That said, Antec is a reliable manufacturer for the most part. I really don't like to put anything smaller than a 500W on a computer these days, because I don't know what power consumption is going to be like two or three mods down the road. As such, I have a monstrous 1KW Ultra--there was a good bargain on and so I picked it up. Now I don't worry about my power supply. Ever. This is what a 70-amp 12V rail will do for you.
If the rig in question really is that old, all you really want is for it to not blow up, and be able to power all your stuff. After that, being able to use it on your next build would probably be nice.
To answer your final question, the Antec pumps out a claimed 500W and 36A spread equally over two 12V rails. The Logisys *claimed* to put out 480W and a measily 17A on the 12V (the 12V, in case your wondering, is the one that all your important and power-hungry stuff runs off of). Funny, I had a 17A PSU once...and it only had a cajones to claim to make 330W.
The Antec, even at more than 3X the price, is clearly the better power supply. It's putting out way more juice than the Logisys, and the company's PSU rep is better.
