If you want to OC high and you dont have a good/new Mobo then buy a X48 (going by reviews).
If you already have a X38 then dont.
I aint seen any reviews that make is that good TBH.
theres absolutely zero evidence that X48's will overclock better than the x38's, 5 review's with cherry picked/made x38 chipsets named as x48's. Anandtech's review hit 500Mhz fsb. my p35 can do that, many people's can, as can many x38's as it is.
terrible advice, why when you can buy a £125 X38 would you pay between £75-125 for probably a very very very VERY slim fsb advantage. In the end 500fsb might not be useful, you might need a higher multiplier lower fsb for the right memory ratio that gives you the best balance in your system. 450mhz worked better with my Q6600 because it let me use a higher mem ratio, at 500Mhz with a drop in multiplier i couldn't keep the memory at higher ratio so memory speed took a large drop and gave worse performance.
A 8800ultra over a GTX is un-seeable speed difference, for massive price hike, but at least you know the difference is there. X38/x48 are rated the same and theres no reason both can't overclock the same, just ones £100 more for entirely no reason. you'd be a sucker to buy one.
P35's overclock Q6600 pretty well, afaik nvidia chipsets, at least the slightly older ones weren't so good for high fsb clocking on quads. A cheapish P35 should do you good, a Gigabyte/asus midrange and up board, or a cheapish X38 should be plenty.