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The cpu your looking to upgrade is no better than your current version, You better to look at the main dual core Athlon range ie 4400 cpu, but you would need to check the BIOS revisions.
I picked up a new e8200 cpu, mobo, 4gb ram for £40 delivered recently, a huge upgrade over yours. Add a new psu for 40 and anetter gpu (used) for £30 and you'll have a much better pc for £110. It might also use less power, helping you recoup some of the cost.
Yeah, most shelf PC's are, just wanted to make sure
I would suggest looking for a second hand mATX motherboard, what's your max budget?
Yeah, if anything you'd want to go trinity however it isn't very cheap and as that is an old PC the RAM will need updating too.