The one set of DIMMs that has impressed me most is Corsair's new XMS2 5400UL modules, because I worked with them extensively while preparing our review of the nForce4 SLI. Corsair and NVIDIA have teamed up to match these DIMMs with the nForce4 SLI Intel Edition platform, complete with an official certification. They're fast enough to take advantage of the chipset's DDR2 667MHz memory clock speeds with very low latency 3-2-2-7-1T timings, a very sweet combination of high speeds and low latencies. In fact, they swallow up 2.1v of juice in order to run at that speed, and the things don't even get all that hot to the touch.
DDR2 memory can also be configured for even higher clock speeds at slightly looser timings, and several manufacturers are taking advantage of that fact. Corsair also just announced its XMS2 6400 modules rated for 800MHz at 5-5-5-12 timings. Similarly, Kingston has announced its HyperX DDR2 PC2-6000 modules, which are tested at 750MHz with 4-4-4-12 timings. The press release claims Kingston hit 866MHz in the lab with these puppies, so the stock rating may be a little conservative (or the press release may be a little overheated.) In fact, the SPD timings of 4-4-4-12 are apparently set to meet JEDEC specs.
Meanwhile on the Athlon 64/DDR front, OCZ has recently unveiled a partnership with DFI that's produced some modules specially tailored (or at least targeted) for LANParty nForce4 motherboards. The PC-5000 Platinum nF4 Special series promises clock speeds of 625MHz at 3-4-4-10 timings for plain ol' DDR memory. Both companies are pushing the extreme overclocking potential of this mobo/RAM combo. Given the high voltages available on the LANParty NF4 boards, up to 4.0V for RAM, I don't doubt they could hit some astounding speeds.