Well I'm planning on taking my old current power supply out of my computer and putting it into one of my older rigs.

I've narrowed it down to two choices but not sure which one to get.

mushkin 550300 800W

or

OCZ EliteXStream 800W

My current rig is listed in my sig the only thing that will change over the next couple of months is I plan on getting a 4870 or a 4870 X2 to replace my old 8800GTS.

Hard to say. I like OCZ but that one doesn't have amperages posted in the specs section.

http://www.newegg.com/Product/ShowImage.as...0W+Power+Supply

Heh didnt't think of that. The Mushkin has 64A total on 12v rail and quad rails as opposed to the OCZ which has a single 62A. I guess it depends if you would prefer quad rails as opposed to a single rail. Might be better to have quad. Though I did have a 400W antec psu that would totally not power my old computer (3.0 pent4 and hd3870) which had dual rails at 14A and 15A but a 300W enermax psu with a single 22A rail could. Not really sure what you should get but both should be adequate for what you want.

I thought have more rails in a power supply meant that it could supply power more efficiently.

Not really...

"Like I said before, typically there is only one +12V source and there is typically no additional filtering stage added when the rails are split off that makes the rails any more stable or cleaner than if they weren't split at all."
-JonnyGuru