i assume thats 1000 in $US

personally i wouldn't go with Dell because i'm getting a new computer and for equivalent specs you'd have to pay $NZ3000+
whereas just buying the parts only cost me $NZ1518.65

a few extra things to consider, is the motherboard, and what it can fit. how much RAM it will take (speed of the ram isn't an issue unless you overclock your processor, or so i'm told), the number of harddrives it can support and how many graphics cards it can have

as for your processor to ram question... i haven't an answer

i dont want to switch to vista either, but for operating systems XP 64bit edition is less stable than vista 64bit or Windows Server 2008 64bit
the advantage of having a 64bit OS is that it can handle larger numbers (the max number is twice as long), that and it can recognise over 3gb ram without having to do any hacking

and by less stable i mean there aren't as many drivers for stuff and stuff doesn't run properly

but why not just use the OS on your current computer on the new one you want? just put the disks into that one