Well the SS51G vs SB51G...
depends how long you can wait and what price you want to pay.....
From the inital reviews...the SB51G should be as fast as the i845GL solutions out there.......The SS51G is slightly slower than the VIA KT333, i845 soultions out there.....but we are talking less than 5% here......
As for stability....They both should work fine within specifications........the SS51G however doesn't operation very well above 133MHz ........FSB...Powersupply wise.....the VCore is abit low on the XPC series......Vcore voltage can only be controlled by PIN wrapping(which is very easy to do)...but not many ppl have noticed any effects on stablity there.....
All the P4 XPC offer Firewire and USB 2.0...................
As for which cube to get...its all up to you...you will probabaly have to wait awhile for the SN41 Nforced based XPC........The SB51G is just comming out...(Anand is goin to do a review of it soon...) the SS51G has been out for some time and probably the cheapest one...