In the real world I never noticed much difference between Athlon Thunderbird boxes with DDR vs. SDR. Maybe it was a side effect of the kind of lousy chipsets all too many Athlons were cursed with at the time (looking straight at you, VIA) or maybe in the real world the theoretical increase in memory bandwidth just didn't translate to much of a gain. (IE, there weren't that many things that were memory-bandwidth constrained.)
I still have fond memories of my 1.33ghz Thunderbird. (It was a 1.33ghz instead of the 1.4ghz because, as was typical at the time, buying one speed grade under the fastest saved you a *ton* of money.) In sheer FPU grunt it stayed competitive with Pentium 4's for a good three or four years after I bought it, and it was *crazy fast* compared to the PIIIs that were its major competition when I bought it. I never quite felt the same love for the Athlon XP; I didn't bother upgrading when it was new because the T-Bird was fast enough, and when I encountered them my impression of them suffered because a lot of them ended up in really cheap, lousy machines. (Compaqs, eMachines, etc.) By that point I'd pretty much drifted into the Evil Empire's camp not because I cared for the P4 (I didn't), but because Intel won me over with the relative quality/reliability of their later motherboard chipsets. (The 865 is about when they started getting it right.)