If you're getting a 333MHz CPU, I suggest getting some PC3200 DDR RAM, then running it at 333 with the CPU. Don't scrimp on the RAM, get some good stuff like Corsair XMS. If you get the latest revisions of the NForce MB's they will be fully compatable with 400FSB processors too, so consider that. I'm running a XP2600(333) here with no problems, but it's also running at 2.3 instead of 2.1 GHz, which is a moderate O/C.
Running in "synch" simply means that you're running the RAM at the same speed as the CPU. This is a lot faster than running it "asynch". An example would be the above situation. If you install PC3200 RAM and let the board set the speed, it'll be running asynch, (400). So you go into BIOS and set it to run at 333, or 100% (synch). Since you're using DDR memory, the actual FSB speed will be 166MHz in the BIOS.
DDR memory is fairly finicky, a lot of what is rated at 333 isn't all that great at that speed, and can cause instability issues. this is why I recommend getting PC3200. Although if you get the good stuff, PC2700 should work well enough too. It's just that getting PC3200 gives you some future compatability with the 400FSB processors.