20K "might" be possible with a single vmodded 4870 if the Deneb performs better clock for clock than Yorkfield AND clocks better. Both of which I doubt...
Here's a bench I did when I had my Q9650 @ 4.2ghz....I should have scored another 200-300 cpu pts I think, but even Server 08 does not compare with XP performance. I think I'm going to have to install an XP OS on another drive for benches.
ORB - Compare
As you can see, it's only just over 18000....this was with the 4850 @ 880/1150.
Another 2k is going to be difficult....i7 can probably do it with a vmodded 4870 but I have not seen anything indicating that Deneb performs close to i7. We shall see.
Crossfire is another story. I could have easily scored over 20k even with 4830s