PSU BLOW OUT

It may or may not have damaged other components. It just depends.

It ran for about 20 mins smelling of burning and then the power went down around the house and smoke came out the back.

Hopefully, like i said earlier, it hasn't destroyed the motherboard and other parts, no visible damage.

What system spec? What make/model old and new PSUs? Don't buy cheapie no-name PSU! They are rubbish and will blow other components. Why did you run your computer with smell of burning?  
At least you know what that means now.  

Mike.

I think you put the new ram incorrectly thats why the PSU blew away after 20 min

I'm more suprised at the fact that you didn't replace the old PSU in the first place...250W is waaaay to weedy for what I'm thinking you'd want to do with it. The fact it went pop when you put in extra RAM just hints at what strain you were putting it before...

It was actually a pre-build system that hadn't been updated before, i never thought to look at the power supply before putting extra RAM in because i've never known it be an issue with RAM to be honest.

I hate pre-build systems now, In my opinion you dont get value for money, most of the big companys use the cheapest options when it comes to PSU units & components,
Years ago I had a pre-build pc and the PSU blew 3 times in 3 years, and I never did any upgrades with that pc, it had a shi`tty 300w, they only give you just enough power to supply all the components inside, so when you think about upgrading any components your fu`cked.
So from now on I allways build my own pc up, with a good PSU with plenty of watts for future upgrading of components,
Roffey123 is right, 250w is way to weedy, I cant help but think the PSU was running right on the limit even before you decided to upgrade the ram, but anyway theres a good chance your PCs components should be ok once the new PSU is in, it might be a good Idea just to start your pc up with your old ram first and if its ok then try putting the new ram back in, and if you have problems then a least you know its only the new ram thats damaged, but this is very unlikely to have happened considering you didn`t have much power going through the components.

Fair enough - sorry if I sounded a bit aggressive towards you. As other people say...take your time after you've installed your new PSU - there's no telling what would of happened to the components after it went pop, the fact the PSU was still running before it eventually burned out suggests that there wasn't a massive spike in voltage...so hopefully everything will be fine.

The only way to know is to plug in the new PSU and boot up.

Just put in a 650w PSU. Everything seems fine. Only problem is that it was slighty too big for the case so im minus a DVD drive. Luckily though it was a freebie from a friend and thank god everything is fine. Just made a fresh backup.

Congratulations