If you have a good Motherboard with DDR then go for the Athlon upgrade. Otherwise you will have to get a new motherboard for the P4a. Again if you want to get the best out of a P4 then you need to get the i850 chipset and new Rdram (starts getting a little expensive).
If you only have standard SDram you may still be able to stick the XP in your existing motherboard, but you may have to update the Bios.
However as people will tell you, it depends on what software you want to run. I.e. is it optimized for the P4. Do you require very high bandwidth (video editing), etc
i got a GA-7DXR mobo oroginally had a 1.2 athlonC oc to 1.466 on it but that died so i replaced with the duron (cheap-just to get my pc back up and running). ideally i will stick with AMD cos they cheap and highly competitive. also i like my mobo! fast stable and plenty of nice features! anyway the P4a seems to hold the performance crown for now. can u guys give me advice on best upgrade using existing mobo or upgrade mobo and P4a?
For an upgrade with a budget, considering that you have pretty good motherboard already, I would just go with a new CPU from AMD and keep all of your other components. Upgrading to a P4a could give you some serious power if you are willing to overclock, but it would also cost a good chunk.
If I were in your position, I would stick with AMD. I wouldn't see the performance gain from replacing the motherboard, RAM, and CPU as worth it from what performance I could have just with a new CPU.
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