Regarding Fortron and Sparkle: There are two different lines of differing quality: The cheap Hi-Q/Power-Q line, and the regular good line with "FSP" in the model number. Avoid the cheapie Hi-Q/Power-Q lines, go instead for an "FSP" designated Fortron/Sparkle, and you won't go wrong.
OP, at this point, I think it's a power problem, because, what else is left? I highly recommend that you double check whether your PSU is a good Sparkle ("FSP" in the model number) or a bad Sparkle ("Power-Q"), and if it's a bad one, replace it with a Fortron/Sparkle FSP350-60PN (a moderately priced unit of good quality that's recently been revised for more 12 V power) and throw the old one away; if it's one of the good ones, see if you can get warranty service on it.
I say, replace your PSU, put your old CPU back in (because swapping to the Duron made no discernable difference), and wait on your 2800+ upgrade for a month or two if funds are too short because of the PSU.
P.S. I'm using a Fortron FSP350-60PN to run a P4C 3.0 @ 3.3GHz (w/ slight VCore bump), 1 Gig of Mushkin Level One @ 1.65V, a 9800Pro, up to 3 hard drives at once, and up to 3 optical drives at the same time (when I'm actively working on archiving my CD collection to MP3, meaning the CPU is also at 100%). It's been running without a hitch for over a year now, so I think a good 350 watter is capable of a lot more than many give them credit for.
The FSP350-60PN happens to be fairly quiet unless it starts to get hot in the case, and the 120 mm fan moves a lot of air even at its quietest speed. (The difference between the Fortron and the Sparkle versions of the 350-60PN is that the Fortron has a "crank-it" dial for the fan which makes it loud all the time if you wish.)