You can try leaving using the RAM at DDR-333 with 2.5-3-3-6.
The worst case scenario is that the system will not post. You would then most likely nrrd to clear CMOS (see Page 28 in the manual for the procedure) this will reset the BIOS to default levels. If you have made changes to the BIOS then before you start note the settings so you can change them once the system reboots after clearing CMOS.
The more probable scenario (if the RAM is not stable that is) is that the system will boot ok but the RAM may not be completely stable. Even if it is stable I would strongly recommend that you test the stability using
Memtest86
Memtest
-Download Memtest86+ here
http://www.memtest.org/ then burn the .iso (after uncompressing) to CD with ImgBurn or burning software (needs to be bootable) OR download the executable for USB if your BIOS allows you to boot the system from USB
-Place the CD in drive (or attach USB drive)
-Reboot computer, change boot order to CD (or USB), save change, reboot.
-Memtest will load and test the module. Each pass takes around 10-15min to complete for 1Gb, run 7-10 passes minimum. Tests 2 and 5 within the pass are most likely to find errors and you can order loops of these tests in the options.
-Stop the test when you're happy that the RAM is stable, reboot back to BIOS
-Remove CD/USB, reset boot order, save changes, reboot.
Here's a short introduction and instruction with screenshot
You may decide that it's not worth the gain, or decide that it's a good introduction into getting a closer understanding of the system.
I would advise as a matter of course that anyone adding RAM, and/or altering timings, follow a thorough testing and stability regime.
Memtest is not 100% effective in catching bad timings or RAM as it doesn't take into account the whole system running under load where small voltage changes in northbridge (MCH) and RAM can influence stability, but it will ensure that the RAM is theoretically capable of running at it's new timings and the RAM is error free.
