Well... 2.5 usually is run at 2.5-3-3 settings (each number refers to period of time the ram 'waits' at some point. The 2.0 ddr usually is run at 2-2-2 timings.
If you improve all 3 timings I have heard of people getting up to 7% performance improvements on their systems. The actual improvement for most people probably isn't quite that high, but don't take my word on it because I'm just passing on info I have picked up from un-audited sources..
I have also heard it said that ddr @ 2-2-2 timings gives a similar boost to increasing a 133 fsb to 142 or so (with 2.5-3-3 ddr timings)... once again... I can't confirm or deny such information.
I think most people would agree that the faster timings give improved performance, but getting pc2400 or pc2700 and overclocking the fsb would give a larger performance boost (if the rest of your system supported it!)