About your first question,i recommend X38 over X48 for 2 reasons:
1_The difference between X38 and X48 isn't much
2_X38 is cheaper.
Also X38 is older than P45/X48 but its still a very good chipset.
About P45,well i don't recommend P45 for Dual ATI cards because P45 supports CrossFire at dual 8x mode which sometimes hurts the performance compared to dual 8x mode.
About your second question,ASUS P5Q(P45 series) have 2xPCI-E 16x slot BUT when you want to CrossFire a card then the speed will be dual 8x.
In other words it has 2xPCI-E 16x slot,it means when you have single card it will operate at 16x but when you add another card then both cards will run at 16x.
Yes HD 4870 is good for 1600x1200 gaming,but if you are planing to get a bigger display and CrossFire later,you may want to get a HD 4870X2 instead of HD 4870.