XP x64 uses the same Kernal as Server 2003 x64 and is a rock solid O/S. Windows 7 benifits from the larger driver support and if you have to buy XP x64 I would recommend getting W7 x64 instead. If you already have XP x64 the go ahead and use it.
So does W7 64 support older harware like the AMD 939 platform(a8n-sli)? Does it fall under W7's UDA or will W7 have a way of using the xp/xp64 drivers I already have?