It sounds like you are using the divider already...... with "Timings = 3-3-3-6 2T @ 187Mhz "
On a 1:1 ratio, you should have 280MHz like your CPU's FSB, based on your 280MHz, there are other ratios such as 10:9 ~ 252MHz; 5:4 ~ 224MHz, etc........you should find the ratio dviders in your BIOS setting for DRAMs.
But the divider method is used for people who's DRAM modules are limiting their OC potential which in your case running 2 x 1GB Corsair XMS Pro DDR550......you should have plenty of MHz to work with.
Setting it to 1:1 ratio with 280MHz would make it slightly over the rated speed but like I said, from your settings, it looks like you're already using the dividers!!!
At 2.8GHz, it's a pretty good overclock already~~~~
But of course, if you can go higher, why not??
Good luck~~