here's what I'd do...
sell the P4 2.8 for 75 bucks right now before everyone realizes conroe
sell the ram for 25 bucks
that's a hundred bucks to start out with to building a new conroe, because once the conroe is released your current parts are going to cost jack~
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otherwise, the P4 you have will be good enough to see a nice upgrade. get an Asus DX SE mobo for 80 bucks, a 1gig kit for a $70 (1.5 gigs totalled with the RAM you have), a much better cooling solution (zalman preferrably) - then overclock the processor to its last bit of stable juice.
200 bucks so far, spend the other 100 bucks on a better video card if you demand heavier loads off of the processor because I know the Dim3000's have integrated Intel graphics or a PCI gpu which isn't all that great compared to an AGP card.
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or get an 805 dualcore with a cheaper priced mobo and just get a 1gig DDR2 kit for a hundred bucks, otherwise there are mobos with support for DDR1 and DDR2 (but not simutaneously running together) you could get a DDR1 512mb stick for 40 bucks to make 1 gig if you transfer you current 512mb stick to match up with the new 512 stick.
805 + mobo = 200
RAM = 40 for a single DDR 512mb stick or about a hundred bucks for a good 1 gig stick
eitherway you're spending 200 or 300 bucks.
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however, keep in mind, that if you decide to just upgrade the memory you are on a win-win situation because you don't have to worry about swapping out the motherboard and worry about PSU costs and other such things like that. your best bet for performance would just be to get two 512mb sticks to make 1 gig and sell the two 256 sticks you already have in your system.
you should know that the PSU and mobo are not compatible with 3rd party hardware according to Dell standards. of everyone who wants you to upgrade the mobo and CPU to something else, you're being told that the PSU and chassis has already been taken care of - so in reality you're really looking at 500 bucks if you're serious.
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upgrade your 256x2 to 512x2 (RAM). get another motherboard; you're looking at getting another case and PSU. RAM is probably the most universal in your situation to see a performance increase. RAM is RAM; supplying your CPU with more resources for productivity.
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finally... you might as well go with a conroe for crying out loud, ultra significant peformance gains, meets beyond the extremes of the Athlon FX 62... and climbing.