Considering that cutting players from the roster will count against this years cap, I can certainly see where the Cowboys will likely trade up. But I'm not sure whittling it down to 6 would be realistic. This list doesn't account for positions the Cowboys could upgrade, such as the backups in the OL, DL, LB, and CB. Those are positions in serious need of an upgrade.
Demarcus Ware can't play every snap and Kevin Burnett, his only back-up just signed with San Diego. Junior Siiavi is not a viable back up for Jay Ratliff. We have 4 CB's, possibly only 3, if we transition Ball to Safety, in a position that typically occupies 6 of the 53 final roster. We have plenty of Safeties, but behind Hamlin and Sensabaugh, an upgrade could present itself in the draft. Your list suggest Greg Ellis will still be with the Cowboys in 09; I'm not so sure. I'd say he's a maybe, considering the youth movement occurring on the roster currently.
Considering the other pitiful replacements we have for an OL that already underperforms, definite upgrades could occur. So, by my count we have about 12 potential roster spots. I'm not saying we will find replacements throughout the draft, but if you consider players such as Pat McQuistan, Cory Proctor, Doug Free, James Marten, Ryan Gibbons, Matt Spanos, Rodney Hannah, Travis Wilson, Paris Warren, Alan Ball, Patrick Watkins, Courtney Brown, Michael Hawkins, Jerome Carter, Steve Octavien, Carlos Polk, Marcus Dixon, Stephen Bowen, Jason Hatcher, Justin Rogers, Tra Battle and the loss of Pacman, Anthony Henry, Zach Thomas, Kevin Burnett, Roy Williams, T.O., Joe Berger, Tank Johnson, and Chris Canty minus the acquisition of Keith Brooking, Matt Stewart, Gerald Sensabaugh, and Igor Alshansky, that leaves about 12 positions, if not more, that need to be filled or upgraded. The only way to determine a filled spot or an upgrade is to create that competition in training camp.
So like I said, I have no problem with the Cowboys grouping a few draft picks here and there to acquire quality over quantity, but within reason.