Depends on your requirements - they have different feature sets. The Gigabyte website has a tool for compairing their mobos and that should give you a good idea. Off course some of the features maybe specific to those mobos and you may see multiple mobos with slightly different features.
x48 is for crossfire at full x16 speeds.. p45 can crossfire but i think it splits the lanes into x8 for both which can be bottlenecked when using stronger graphics cards..
however i think p45 allows higher amounts of ram to run at higher speeds or something.. but that's kinda overkill anyways cuz a good 800/1066 set can get you far.. or maybe it was higher fsb with more ram.. anyways, if you're gonna crossfire, get x48, if you want a single video card then go p45
also, these are chipsets for lga775 if you didnt know..
x48 at this point in time is a waste of money. Besides being a rebranded x38 that is way more expensive with no more functionality, like Shadow said LGA 775 is dead. Core i5 will be officially launched in about a week or so, so just hold on for a little.