I have kinda the same setup at home. I'm using an xp-m 2800+ Barton. Point is you should look to get out more fsb on the proc more than pushing your multiplier should get you more performance. As for your multiplier problem the only thing that could solve it would be a bios update but even then...
Just as a hint my Proc runs @ 12,5x400 @ 1,725V - Also requires bis cooler (got Zalman 7700Cu adapted on it)
I used to run the same setup as u did but for "prestige" reasons i switched over to a nf7 abit board and a rare 7900gt agp card. It s my old kicking Lan pc
Runs even crysis @ 1280 with medium settings