P45 has only been out for less than a month, just give it sometime, there is no reason it should have any limitation with hd 4850 using x8 bandwidth on each slot, and there is no way in hell its maxing that out, it is probably driver related or the particular board they were using, I got a p5q-e myself fo 160 out the door, just becuase it burns less power and heats up less than the x38 or x48.
well if you have a 1920x1200 monitor than it should not really matter the only significant difference I saw was at the highest resolution they were testing at.
if two times 8800gtx's or even ultra's were not limited by x8 bandwidth and that was on the first gen slots, and the second gen slots should not really limit the hd 4850 by any real means. Ati actually does optimize for the new chipsets because its not their in-house chipset, so the next catalyst or the one after that should take care of that.
if you are using a monitor with 1920x1200 get the p5q-e its a great board and on an initial release bios I am running my e8400 at 4.2ghz, if you are using the 30inch monitor than by all means go with the x48.
it all depends on your needs, I did not wanna spend extra money on the x48 since the new chipsets and the native quads will be coming out at the end of the year.
Yes over P35 the P45 will be a significant difference when it comes to performance. I might crossfire once the hd 4870 comes out. I believe there was another review at bi-tech, where the p5q board was outperforming the x38 or x48 in crossfire, I am not really sure though, Asus has a good history of having their bios refined for dual card setups.
And don't just go by one review, I wish Anandtech did a comarison for the two boards and I am sur they will try to sort out if and why the P45 would perform less and they would actually go in detail about the performance issues if there are any, Tweaktown just did a crossfire X with 4 hd 4850's and where two of the cards were limited by x4 bandwidth, I would rather see them do it on a phenom system, Crossfire does not like x4 slot, I had a nightmare in a few games including my previous p35 chipset, ofourse I have sold of my hd 3870's and now I am waiting on the hd 4870 reviews.