XP by itself won't recognize the full 4gb of memory. Make sure you use a 64bit OS to address all of it.
Nice system
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Asus M3a79-T Deluxe
2gb Crucial 8500 ddr2
Sapphire Radeon HD3870-
(2)WD 360gb sata2-
Phenom 9950BE-
Thermaltake Shark-
Logitech Z5500-
Acer 22" LCD 5ms
Corsair 850w ps
...and a partridge in a pear tree...