This may interest you then. I had to replace a 150W PSU only yesterday. To be honest I hadn't realised it was only 150, had never given it a thought. That machine has only ever been turned off about 6 or 7 times in three years. It's in use every day and has never missed a beat 'til yesterday. It's only a 750 Duron with spec similar to what you describe except only 256 RAM but has been a wonderfully reliable machine so I don't think there is much to worry about.
I thought I might have to get a larger case but managed to squeeze in a 350W replacement after removing a blanking plate.
My local shop had only got the 350's in stock but he had some cases and if nec I would have bought one and bunged everything in there. Just another idea but if the 150 is OK I'd leave as is for the mo. Could have years left in it yet!
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