It depends on the resolution you are using as well, you start to see the impact of the lower bandwidth of 2x8 vs 2x16 at 1600x1200, with significant impact starting at 1900x1200 and higher. Do a google search on 4850 Crossfire reviews, there is one done by a site that tested for this with a P45 board vs an X48 board.
See someone beat me to it. The article shows it well.
Another vote for Gigabyte...
I'm running a GA-P31-DS3L
It's cheap and very overclockable.....a little short on features but suits my needs.
I would have got a GA-P35-DS3? but wanted to keep my upgrade as cheap as possible.
If your going to use a 45nm cpu last time i looked on newegg they had asus p5k pro for $109. They work with the new processors quite well. mine is doing prime95 as I write this. E8400 4ghz/2000fsb. been going for 2.5 hrs..... max temp 63 deg. (real temp 2.60)(air cooling) 1.18 vcore. 1.47 fsb. 6 sata ports- crappy audio (curse of onboard audio )
The gigabyte boards are fantastic too.
They all seem to be pretty flexible these days.
If you are going to purchase two video cards then why not get only one now and spend more on a better crossfire/sli board and get the 2nd card when you can afford it? Especially if buying oem license operating system.
If you already have the cards then disregard above statement.......