How many watts is it?
It's good to have at least 500W power supply. Oh, and it should be heavy. If it weighs nothing and is priced at $25, you're not getting what you think you're getting. A decent one should be between $75 and $120.
If you want to cut down on the sound you can replace the fans. Fans are relatively cheap.
Chances are your power supply can run for a pretty long time. If one of the caps goes bad though your computer probably won't like the huge increase in ripple voltage. I don't know if it would cause damage, I'm thinking it would just make your computer not run.
I've no idea of the specs or the weight of the power supply. I got it when some guy built my first decent system, back before I knew how to build my own, and it's never been taken out of the case. It's just generic grey on all the visible exterior parts, and I think I'd have to remove it to find any labels on it, which would mean removing pretty much every other component from my case first.
I'll need a fancy screwdriver to replace my main system fan; I'm not sure if I've got one that will work. The screws are one of those exotic designs that stop people from removing them unless they have equally exotic tools.
My current setup:
Power supply (6 years old, lots of dust on the fan blades)
Large case
MSI KT4V motherboard
Athlon 2600XP processor
1GB of DDR PC2700 RAM
Raedon 2600 AGP graphics card
1x30GB IDE 5200rpm hard disk
1x200GB IDE Seagate 7600rpm hard disk with 8MB cache
1x400GB SATA Seagate 7600rpm hard disk with 16MB cache (connects through a PCI SATA card, and for some reason stops my comp from recognising a floppy drive)
1xCD-RW (only got power connectors for 4 internal drives, and only got motherboard connectivity for 4 IDE drives)
Soundblaster Audigy value PCI card (not that it matters; my speaker system is very basic)
Firewire PCI card
It's a system that I've generally been very pleased with. It used to hang and require a hard reboot several times a day, but that turned out to be due to interference from other electronic equipment nearby, and was resolved when I moved the offending equipment a bit further away. Since then, it's been rock solid. It's gotten to the point though where it's not really worthwhile upgrading most components, since they wouldn't be transferable to a new motherboard. I'm not sure if PCI cards would work in PCI Express, but I'm pretty sure an AGP graphics card wouldn't, and PC2700 RAM wouldn't do me much good in a more recent motherboard, so the only things I've upgraded recently are the hard disks.
Also, does anyone else fear and dread the installation of heat sinks? I find them insanely hard and nerve-wracking to get on, and feel like I'm a hair's-breadth from cracking the motherboard or crushing the processor when trying to fit them.