Dead Computer (EDIT: NOT DEAD!!! ^_^)

[Wiregeek]

I would not be comfortable or confident if I was running my PSU at over 90% load...

http://hardocp.com/articl..._look_at_power_supplies/

Read this. Pay attention to ripple, load, and ambient temperature.

Please size your PSU properly.
"No can brain today. Want cheezeburger."
From NGE: Nobody Dies, by Gregg Landsman
http://www.fanfiction.net/s/5579457/1/NGE_Nobody_Dies

[Jinx999]

I agree. You are pushing it with that close to the limit of the PSU. IIRC, I use a 700 watt PSU with roughly similar load required. I've seen a review of no brand PSUs that fail immediately at that kind of load compared to their rated load. When you see me rolling my eyes at excess PSUs, that's been people recommending 900 watt units for normal PCs.
Edit: Actually 600 watt PSU with 400 watt recommended

[Wiregeek]

heh. Newegg's calculator says 384watts for my system, and 650 for my planned upgrade, I'd say like 75% of that is the change from a 9800 gt to a GTX 580
"No can brain today. Want cheezeburger."
From NGE: Nobody Dies, by Gregg Landsman
http://www.fanfiction.net/s/5579457/1/NGE_Nobody_Dies

[Logan Darklighter]

I may have made some kind of mistake, too on the calculator. 

Thing is - I haven't made that much in the way of mods to the computer. I took out the dial-up modem card when I first got it and put in a wireless card in the bottom-most slot. And really? That was all I've done until now. The latest video card is the first real major upgrade I've done. And that took it from an Nvidia GeForce GT230 to a GeForce GTS 450. The 500w power supply exactly duplicates what the computer had originally. 

How much of a difference is that in terms of power/heat? Any way to tell? 

[Wiregeek]

50w more according to NewEgg's calculatior, grain of salt is that it does not have the GT230 - I used the GT240 instead.

Your original PC vendor sized that power supply to run at ~85% load at all times, since they are cheap bastards and care nothing for you. You very likely exceeded the sticker rating of your power supply when you changed video cards.

And if you read the HardOCP article I linked, sticker rating isn't all...
"No can brain today. Want cheezeburger."
From NGE: Nobody Dies, by Gregg Landsman
http://www.fanfiction.net/s/5579457/1/NGE_Nobody_Dies

[Logan Darklighter]

Well bottom line. Is this thing likely to last? For how long? Under what kind of load should I expect trouble if any?

Earlier in the thread, I posted a link to a review on the Acer M5800. That was my computer exactly as it was when I got it. With new power supply and video card. What's it look like it's doing now?

[Sofaspud]

Bottom line?  Nobody can predict how long it will last.
At your current load-vs-limit scenario -- and keep in mind the heat factor, which is dependent not only on ambient temperature, airflow, humidity, and also dust and how much 'effort' you're making the box go to (gaming, for example), and which dramatically affects power consumption -- you will have a failure, and it will likely be related to the PSU.  If you need expansion on this point, see Exhibit A, the same box pre-"It's Dead, Jim".
If you want my experienced and somewhat cynical, IMO-only take on it... it'll fail somewhere between ten and thirty days after the warranty on the PSU expires .  Typical warranties are for 90 days or a year, depending on manufacturer.  Some don't have one at all.  You have to look at the docs that came with it.
Whether it takes your mainboard, memory, CPU, or graphics card with it is the question.  Again IME, the fail order is usually mainboard->graphics->memory->CPU, once the PSU starts dying.
I think I'm going to try and take pictures tonight of my poor little breadbox, the one with the PSU That Could(n't).  As I said, it lasted for a good long time, until things started mysteriously failing and I opened it up to discover scorch marks.

--sofaspud
--"Listening to your kid is the audio equivalent of a Salvador Dali painting, Spud." --OpMegs

[CattyNebulart]

It also depends on how 'clean' your power is when it comes out of the wall, because the power you are getting is not constant but has surges and other noise in it, and those tend to put additional strain on your equipment and if your PSU is no good will pass that noise to the rest of the components. A UPS typically does a lot to condition power and so it will tend to make your PSU last longer, keeping the PSU cooler and less humid will also extend life, etc. Also running at less than full rated load is easier on the power supply since all the parts are rated for more than that, up until a certain point where the chronic underload becomes more of a problem.

In short the lifetime is very hard to predict and it's influenced by a lot of factors.

If you expect to use power in the 400-500W range you probably want a power supply in the 600-800W range, or a very high quality one. You want at least a 20% margin but you also don't want a situation at where you'll frequently be bellow 30% load (for efficiency reasons, it is at this point that most power supplies will rapidly lose efficiency), though good power supplies should be fine until you start hitting the 20% or less load consistently.
E: "Did they... did they just endorse the combination of the JSDF and US Army by showing them as two lesbian lolicons moving in together and holding hands and talking about how 'intimate' they were?"
B: "Have you forgotten so soon? They're phasing out Don't Ask, Don't Tell."