What's better: Single rail 18 or dual rail 16+17?

don't skimp on your power supply ever it will lead to more problems in the future always buy quality or it will all end in a puff of smoke ! Your best bet is to do a full upgrade if finances allow. The artefacts you are seeing could be your gpu is on it's way out due to sub standard power supply

I think what your forgetting here, even if you have a PSU with two 12v rails at 16 amps
=32 amps combined is this the 6800 Gs needs a steady 18 amps from the 12v rail.
In short your running out of amps you need a higher amp rating on the 12v rails.

Well, it isn't the graphics card going bust since using the 450w PSU solves the problem. BUT it makes the computer noticeably slower, and sometimes disk does not seem to work (but it's turned on.)

For instance, i always need to open 2 firefoxes for them to open. if i open one it will not open. I might be saying crap, but is it possible that the ripple / noise is the culprit?.

Your post is somewhat confusing: Are you using only one of the two 12v rails? Or have you connected them in parallel? If the latter, does the documentation say that you can do this?

You list a number of devices: What is powering what? I assume that the older PSU has other output rails as 18x12 = 212W, which is a pretty inefficent PSU!

The bottom line is this: Run anything at near 100% capacity - Regardless of what the specs say - And it will struggle, and its life will be reduced. If it is true that your GPU requires 18A and it is only connected to one rail then the graphics artefacts you are seeing are probably due to due to overloading. If you have commoned the two rails, and you shouldn't have, they could be due to the PSU being used inappropriately. Either way, if your GPU isn't "going bust" now, it soon will be if it is being supplied from an overloaded power rail that could be oscillaitng due to being over-current. If the rail fails under high demand (Minecraft) then you are clearly sailing too close to the wind.

Since you list a whole load of items, including "some other small things" (are you SURE they are small?) and I am not clear on what is being powered off what, the only way to be sure is to get hold of a multimeter and measure what is actually being drawn. For long-term reliabilty, I would restrict the loading on a PSU to 70% of its quoted maximum, and run it strictly in accordance with its specifications.

OK ... 216W!!!

Yes i'm sure, small things such as PS/2 keyboard, USB sticks and whatnot. But the thing is: the one that brings trouble is the DUAL RAIL 500W. The 450W single rail works just perfectly. The only thing that confuses me is how a PSU can affect performance and disk activity...

and there are more outputs, some 25A for the 3.3 and 5V each, 2A for the standby and 0.8 for the -12V (curiously, the label says -5V but i know it's -12V since -5V was extinct in ATX v1.3 and this is an ATX v2.0 PSU. Just shows how good these brands are...

further more, i noticed the fan was operating badly, so i sent it to a repairsman to replace the fan and now i notice the voltage came up from 11.2 to 11.4 and the unit and the computer are much cooler.

the outputs above are for the 450w single, the 500w dual are as follows:

3.3 - 34A
5V - 40A
12V1 - 16A
12V2 - 17A
5VSB - 2.5A
-12V - 1A

rated at 500w with new design (fan on the bottom, 140mm, mesh around the socket) while the other one is the older/OEM design (fan next to the socket, 80mm)

and sorry for the triple post but i can't edit posts.

Hmm. This is a tough one, but I note with interest what you say about the fan. Is it the case that both the fan and the PSU have been replaced and now you are not seeing the problem? If your old fan was struggling then the rise from 11.2V to 11.4 V is entierly explainable. Silicon always draws more amps when it is hotter so it would have pulled the voltage lower. It is making me wonder if perhaps the perceived problem with the 500W is actually a thermal issue. You clearly have quite a bit of kit in there. Could it be that the bigger PSU is kicking out just that bit more heat and things are getting too hot in there and something goes into limit? Is the airflow getting everywhere it needs to go? The fan has changed position, so the airflow will be different. It should be an easy theory to test - Just run the system with the case opened up and direct a desk fan or something into it. If you can cane it and it doesn't fall over, then maybe you just need more ventilation.

the repairsman said the old fan was in fact struggling. The man showed it "plugged" to a generic 12V wall adapter and i saw the fan having trouble to start. He showed a brand new one and it was spinning wild and free (it even made "airflow noise") I couldn't believe THAT was in my power supply. I also couldn't believe the difference. Now my pc sounds like a jet plane (pun not intended) but i come to the conclusion that was normal in these P4-derived computers...

Realize something, is that the repaired one was the 450w.

Going back to the subject of ripple and noise, yes, it can cause all sorts of subtle effects. Plus a worn out fan could generate its share of spikes and electromagnetic interference. I think you really need to check that all of your power rails are operating comfortably within their individual current limits, and that the total output rating isn't exceeded. If nothing is being stressed, you could try playing around with the routing of cables (if you can) in case something is picking up interference.

That's the problem. I can't. It's a non-modular (aka "octopus") style PSU and the 20+4 ATX cable is too short. It actually needs to be fitted between the HSF and the PSU case. With the other one, retaining the non-modular style, the ATX cable is much longer and can go round. But on the other hand it stands close to the GPU, which might be the culprit of the artifacts on the GPU as well.

It could well be. Other than that, i'm stuck for further ideas. Sorry I cant be of more help.