Most powerful processor for that motherboard was the Athlon 64 3700+. That's a single core processor with DDR ram. Today, dual core with DDR3 ram is roughly 10 times the power of the Athlon 64 and is pretty much the entry level. Windows XP is an no longer a supported operating system, this means any threats identified and patched on newer OSes are not fixed on Window XP, so it's something I would not want connected to the internet.
I, too, would heartily recommend a complete new system.