Mushkin was totally dominating the 2x2 GB RAM scene early on with kits using Micron D9GSV. (Like the one you found.)
When i say dominating, i mean it.
No other company had anything even remotely close, since no other ICs at the time could clock anywhere nearly as well.
But then Micron quit making D9GSV, & Powerchips was discovered to clock well & was far cheaper.
So while all those older kits are still listed on their site, they don't make them anymore.
They also happen to use far more expensive chips, so usually you're lucky to find any of the older kits for close to $300+
All the newer Mushkin kits use Powerchips, like everyone else, which clocks to around DDR2-1100 speeds, maybe a bit more if you get lucky.
Those older Mushkin D9GSV would easily blow that away, but it took a lot more voltage, & once again, they're not really available anymore.