That 64-bit CPU can be 100 Watts of your power budget and you will want another 100 as "spare" to keep out of trouble. Here's why...
http://www.tomshardware.com/howto/20040122/
http://www.tomshardware.com/howto/20021021/
Next you'll need a super case to lat the heat out.
The next problem. XP and drives over 127GB will not partition past 127GB as ... XP doesn't support such until SP1. If you can slipstream SP2 into your XP OS, then you can avoid many issues. Look up discussions about AUTOSTREAMER in these forums or google.com
The final problem is that the newer Windows OSes don't transplant from machine to machine. You can find numerous posts where people don't have a backup of what they can't lose and move the drive only to discover... Where did it all go.
Bob
An old system died and I am replacing the CPU MOBO and memory.
CPU= Athlon 64 3000
MOBO= ASUS K8V SE Deluxe
Memory = 2 512 meg DDR 3200
400 Watt Power Supply
I am going to hook up a WD 200 gig HD that was in the old system.
This Drive is partioned with Windows XP on the C: Drive.
Can you give me any ideas on what problems I will run into and how I can avoid them?

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