although sp2 will overwrite sp1, but I will not recommend using 64-bit xp now, as you;'ll find your applications getting slower, also drivers wont be available for all the hardware, 32-bit application tend to perform slower when used on 64-bit o/s. rest your choice, as I still havent used the 64-bit, despite I went to buy the 64-bit, but later on changed my mind.
Intel Core2Quad 2.66Ghz 12MB L2-Cache
4096MB DDR Kingston Hyper-X
XFX 9600GT AlphaDog
Seagate 2 X 320GB SATA2
LG L246WH Flatron 24"